Patent Publication Number: US-11047718-B2

Title: Physical quantity measurement device including a passage flow channel having an inner surface with a height narrowing surface

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of international Patent Application No. PCT/JP2018/010139 filed on Mar. 15, 2018, which designated the U.S. and claims the benefit of priority from Japanese Patent Application No. 2017-079877 filed on Apr. 13, 2017. The entire disclosure of all of the above applications are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a physical quantity measurement device. 
     BACKGROUND ART 
     As a physical quantity measurement device for measuring a physical quantity of a fluid, for example, a physical quantity measurement device for measuring a flow rate of an intake air taken into an internal combustion engine is known. The physical quantity measurement device has a discharge passage for discharging the inflow fluid, and a branch passage branched from the discharge passage, and a flow rate detection unit is provided in the branch passage. While the branch passage is curved to make one turn, the discharge passage is not greatly curved. In this example, a foreign matter having a relatively large mass of the foreign matter flowing into the discharge passage together with the fluid tends to move linearly as compared with the fluid. For that reason, the foreign matter having the relatively large mass tends to be discharged from an outflow port of the discharge passage without entering the branch passage. As a result, the detection accuracy of the flow rate detection unit is inhibited from being lowered by the presence of the foreign matter. 
     SUMMARY 
     One aspect of the present disclosure is a physical quantity measurement device that measures a physical quantity of fluid. The physical quantity measurement device includes a passage flow channel that has an inflow port and an outflow port, the fluid entering the passage flow channel through the inflow port and exiting the passage flow channel through the outflow port, a branch flow channel that branches off from the passage flow channel, and a physical quantity detector that detects a physical quantity of the fluid in the branch flow channel, wherein a boundary between the passage flow channel and the branch flow channel is defined as a flow channel boundary portion, a pair of opposing surfaces in an inner peripheral surface of the passage flow channel face each other across the flow channel boundary portion and the outflow port, a direction along which the pair of opposing surfaces are arranged is defined as a width direction, a direction orthogonal to the width direction and orthogonal to an inflow direction of the fluid through the inflow port is defined as a height direction, the inner peripheral surface of the passage flow channel includes a height narrowing surface that extends between the pair of opposing surfaces at a position downstream of the flow channel boundary portion, the height narrowing surface narrowing the passage flow channel so that a height dimension of the passage flow channel along the height direction is gradually reduced toward the outflow port while reserving a parallel region extending linearly between the inflow port and the outflow port parallel to the inflow direction in the passage flow channel, and the height narrowing surface is not exposed through the inflow port in the inflow direction. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The above and other objects, features and advantages of the present disclosure will become more apparent from the following detailed description made with reference to the accompanying drawings. 
       The above and other objects, features and advantages of the present disclosure will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings: 
         FIG. 1  is a front view of an air flow meter in a state of being attached to an intake pipe as viewed from an upstream side according to a first embodiment. 
         FIG. 2  is a cross-sectional view taken along a line II-II of  FIG. 1 . 
         FIG. 3  is a diagram a periphery of a passage flow channel in  FIG. 2 . 
         FIG. 4  is a diagram illustrating a configuration in which a ceiling surface of the passage flow channel does not have a step surface, unlike the first embodiment. 
         FIG. 5  is a front view of an air flow meter attached to an intake pipe as viewed from an upstream side according to a second embodiment. 
         FIG. 6  is a view showing a configuration of a housing main body in a state in which a back cover in  FIG. 5  is removed. 
         FIG. 7  is a view showing a configuration of a housing main body in a state in which a front cover in  FIG. 5  is removed. 
         FIG. 8  is a diagram the periphery of a passage flow channel in  FIG. 6 . 
         FIG. 9  is an enlarged view of the periphery of a step surface. 
         FIG. 10  is a diagram illustrating a configuration in which a ceiling surface of the passage flow channel has no inflow step surface, unlike the second embodiment. 
         FIG. 11  is an enlarged view of the periphery of a step surface in a modification B1. 
         FIG. 12  is an enlarged view of the periphery of the step surface in the modification B1. 
         FIG. 13  is an enlarged view of the periphery of a step surface in a modification B3. 
         FIG. 14  is an enlarged view of the periphery of a step surface in a modification B4. 
         FIG. 15  is a diagram of the periphery of a passage flow channel in a modification B5. 
         FIG. 16  is a diagram illustrating a configuration in which the ceiling surface of the passage flow channel has no outflow step surface, unlike the modification B5. 
         FIG. 17  is a diagram of the periphery of the passage flow channel in a modification B6. 
         FIG. 18  is a diagram of the periphery of a passage flow channel in a modification B7. 
         FIG. 19  is a diagram of the periphery of a passage flow channel in a modification B8. 
         FIG. 20  is a diagram of the periphery of a passage flow channel in a modification B9. 
         FIG. 21  is a diagram of the periphery of a passage flow channel in a modification B10. 
         FIG. 22  is a diagram of the periphery of a passage flow channel in a modification B11. 
         FIG. 23  is a diagram of the periphery of a passage flow channel according to a third embodiment. 
         FIG. 24  is a diagram of the vicinity of an inflow port of an air flow meter as viewed from an upstream side. 
         FIG. 25  is a diagram of the vicinity of an outflow port of the air flow meter as viewed from a downstream side. 
         FIG. 26  is a diagram illustrating a configuration in which the air flow meter has no inflow restriction portion, unlike the third embodiment. 
         FIG. 27  is a diagram of the periphery of a passage flow channel in a modification C1. 
         FIG. 28  is a diagram of the periphery of a passage flow channel in a modification C2. 
         FIG. 29  is a diagram of the periphery of a passage flow channel in a modification C3. 
         FIG. 30  is a diagram of the periphery of a passage flow channel in modifications C4 and C5. 
         FIG. 31  is a diagram of the periphery of the passage flow channel in the modification C5. 
         FIG. 32  is a cross-sectional view of a passage flow channel as viewed from a bottom side toward a ceiling side in a direction orthogonal to a height direction in a modification C6. 
         FIG. 33  is a diagram of the periphery of a passage flow channel according to a fourth embodiment. 
         FIG. 34  is a diagram illustrating an traveling direction of a large foreign matter. 
         FIG. 35  is a diagram illustrating a configuration in which a partition top portion is exposed to the upstream side from an inflow port, unlike the fourth embodiment. 
         FIG. 36  is a diagram of the periphery of a passage flow channel in a modification D1. 
         FIG. 37  is a diagram of the periphery of a passage flow channel in a modification D3. 
         FIG. 38  is a diagram of the periphery of a passage flow channel in a modification D4. 
         FIG. 39  is a diagram of the periphery of a passage flow channel in a modification D6. 
         FIG. 40  is a diagram illustrating how a large foreign matter advances. 
         FIG. 41  is a diagram of the periphery of a passage flow channel in a modification D7. 
         FIG. 42  is a diagram of the periphery of a passage flow channel according to a fifth embodiment. 
         FIG. 43  is a diagram of the vicinity of the inflow port of the air flow meter as viewed from the upstream side. 
         FIG. 44  is a diagram illustrating an inflow region and a lateral region. 
         FIG. 45  is a diagram illustrating how a large foreign matter advances. 
         FIG. 46  is a diagram of the periphery of a passage flow channel in a modification E3. 
         FIG. 47  is a diagram illustrating a positional relationship between the inflow region, the lateral region, and an guiding surface. 
         FIG. 48  is a diagram of the periphery of a passage flow channel in a modification E4. 
         FIG. 49  is a diagram illustrating a positional relationship between the inflow region, the lateral region, and the guiding surface. 
         FIG. 50  is a diagram of the periphery of a passage flow channel in a modification E5. 
         FIG. 51  is a diagram illustrating a cover portion in a modification E6. 
         FIG. 52  is a diagram illustrating how a large foreign matter advances. 
         FIG. 53  is a diagram illustrating how a large foreign matter advances in a modification E7. 
         FIG. 54  is a diagram of the periphery of a passage flow channel in a sixth embodiment. 
         FIG. 55  is a cross-sectional view of the passage flow channel as viewed from a bottom side toward a ceiling side in a direction orthogonal to the height direction. 
         FIG. 56  is a diagram illustrating the inflow region and the lateral region. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, a plurality of embodiments of the present disclosure will be described with reference to the drawings. Incidentally, the same reference numerals are assigned to the corresponding components in each embodiment, and thus, duplicate descriptions may be omitted. When only a part of the configuration is described in each embodiment, the configuration of the other embodiments described above can be applied to the other parts of the configuration. Further, not only the combinations of the configurations explicitly shown in the description of the respective embodiments, but also the configurations of the plurality of embodiments can be partially combined with each other even if the combinations are not explicitly shown if there is no problem in the combination in particular. Unspecified combinations of the configurations described in the plurality of embodiments and the modification examples are also disclosed in the following description. 
     In a conventional physical quantity measurement device as described above, even if the foreign matter easily travels linearly, the foreign matter does not necessarily enter the branch passage, and there is room for an improvement in the configuration in which the foreign matter does not enter the branch passage. In other words, there is room for an improvement in the configuration for reducing an arrival of the foreign matter to the physical quantity detector such as the flow rate detection unit. 
     First Embodiment 
     An air flow meter  10  shown in  FIGS. 1 and 2  is a physical quantity measurement device that measures a physical quantity such as a flow rate, a temperature, a humidity, and a pressure with respect to a fluid such as air. The air flow meter  10  is mounted on a vehicle having an internal combustion engine  11  such as an engine. The internal combustion engine  11  has an intake passage  12  and an exhaust passage, and the air flow meter  10  is attached to the intake passage  12 . In that case, the fluid to be measured by the air flow meter  10  is an intake air flowing through the intake passage  12 . The intake air is a gas to be supplied to a combustion chamber of the internal combustion engine  11 . The air flow meter  10  is disposed on a downstream side of an air cleaner in the intake passage  12 . In that case, in the intake passage  12 , the air cleaner is at the upstream side and the combustion chamber is at the downstream side relative to the air flow meter  10 . 
     The air flow meter  10  is detachably attached to an intake pipe  12   a  defining the intake passage  12 . The air flow meter  10  is inserted into a sensor insertion hole  12   b  provided to penetrate through a cylindrical wall of the intake pipe  12   a , and is at least partially positioned in the intake passage  12 . The intake pipe  12   a  has a flange portion  12   c  extending from the sensor insertion hole  12   b  toward an outer peripheral side. The flange portion  12   c  extends along a peripheral portion of the sensor insertion hole  12   b , and is, for example, ring-shaped. A tip end face of the flange portion  12   c  extends in a direction orthogonal to a center line of the flange portion  12   c . In that case, the tip end face of the flange portion  12   c  extends in a longitudinal direction of the intake passage  12 , that is, in a direction in which an intake air flows in the intake passage  12 . 
     The air flow meter  10  includes a housing  21  and a flow rate detection unit  22 . The housing  21  is made of, for example, a resin material or the like. In the air flow meter  10 , since the housing  21  is attached to the intake pipe  12   a , the flow rate detection unit  22  is brought into a state in which the flow rate detection unit  22  can come into contact with the intake air flowing through the intake passage  12 . The housing  21  has a flow channel forming portion  24 , a fitting portion  25 , an O-ring  26 , a flange portion  27 , and a connector portion  28 . 
     The flow channel forming portion  24  defines flow channels  31  and  32 . The flow channels  31  and  32  are provided by an internal space of the flow channel forming portion  24 , and introduce a part of the intake air flowing through the intake passage  12  into the housing  21 . The passage flow channel  31  penetrates through the flow channel forming portion  24 , and an upstream end portion of the passage flow channel  31  is referred to as an inflow port  33   a , and a downstream-side end portion of the passage flow channel  31  is referred to as an outflow port  33   b . The measurement flow channel  32  is a branch flow channel that branches off from an intermediate portion of the passage flow channel  31 , and has a curved portion to circulate around the inside of the flow channel forming portion  24 . However, the measurement flow channel  32  does not make one turn, and a portion close to the upstream end portion and a portion close to the downstream end portion of the measurement flow channel  32  do not overlap with each other in the width direction of the flow channel forming portion  24 . Also, the passage flow channel  31  and the measurement flow channel  32  do not overlap with each other in the width direction of the flow channel forming portion  24 . 
     The downstream end portion of the measurement flow channel  32  is opened similarly to the downstream-side end portion of the passage flow channel  31 , and the downstream-side end portion is referred to as a measurement outlet  33   c . The measurement flow channel  32  branches toward the downstream end portion, and thus has two measurement outlets  33   c , and those measurement outlets  33   c  are disposed laterally at positions spaced apart from each other in the width direction of the flow channel forming portion  24 . As described above, because the passage flow channel  31  and the measurement flow channel  32  do not overlap with each other in the width direction of the flow channel forming portion  24 , each of the measurement outlets  33   c  and the outflow port  33   b  do not overlap with each other in the width direction of the flow channel forming portion  24 . The intake passage  12  may be referred to as a main passage, and the passage flow channel  31  and the measurement flow channel  32  may be collectively referred to as a secondary passage. The measurement outlet  33   c  corresponds to a branch outlet. 
     The fitting portion  25  is a portion that is fitted into the sensor insertion hole  12   b  through the O-ring  26 . The O-ring  26  is a member for sealing the intake passage  12  and the outside of the intake pipe  12   a . The O-ring  26  is externally fitted to the fitting portion  25 , and is interposed between the fitting portion  25  and the sensor insertion hole  12   b  in a state of entering the inner peripheral side of the flange portion  12   c . The flange portion  27  is disposed on a side opposite to the flow channel forming portion  24  across the fitting portion  25 , and covers the sensor insertion hole  12   b  from an outer peripheral side of the intake pipe  12   a . The flange portion  27  is caught by the tip portion of the flange portion  12   c  of the intake pipe  12   a  to restrict the housing  21  from excessively entering the intake passage  12 . The flange portion  27  has a flange surface  27   a  which faces the flow channel forming portion  24 . The flange surface  27   a  extends in parallel with the tip end face of the flange portion  12   c , and is put on the tip end face of the flange portion  12   c.    
     The connector portion  28  surrounds multiple terminals. A plug portion is inserted into the connector portion  28 . The plug portion is provided at an end portion of a connecting line electrically connected directly or indirectly to an engine control device such as an ECU, and mates with the connector portion  28 . 
     The flow rate detection unit  22  is a thermal type flow rate sensor using, for example, a heat generation unit such as a heat generating resistive element or a heater unit and a detection surface of the flow rate detection unit  22  is formed of a membrane. The flow rate detection unit  22  is disposed at an intermediate position of the measurement flow channel  32 . When the housing  21  is attached to the intake pipe  12   a , the intake air flowing through the measurement flow channel  32  is supplied to the flow rate detection unit  22 . The flow rate detection unit  22  is electrically connected to the multiple terminals provided in the connector portion  28 . The flow rate detection unit  22  outputs a sensor signal corresponding to the intake flow rate and corresponding to a flow rate of the air flowing through the measurement flow channel  32  to the engine control device as a flow rate signal. The flow rate detection unit  22  detects the flow rate of the intake air flowing in the intake passage  12  by detecting the flow rate of the intake air flowing in the measurement flow channel  32 . The flow rate detection unit  22  corresponds to a “physical quantity detector” that detects the flow rate of the intake air as a physical quantity. Further, the flow rate detection unit  22  is not limited to the thermal type flow rate sensor, and may be a movable flap type flow rate sensor, a Kalman vortex type flow rate sensor, or the like. 
     The air flow meter  10  has a temperature detection unit for detecting a temperature and a humidity detection unit for detecting a humidity in addition to the flow rate detection unit  22 . The temperature detection unit and the humidity detection unit are provided on an outer peripheral side of the housing  21 , and output sensor signals corresponding to the temperature and humidity of the intake air flowing through the intake passage  12  as a temperature signal and a humidity signal. For example, the air flow meter  10  has a support for supporting those detection units on the outer peripheral side of the housing  21 , and the support is fixed to the housing  21 . 
     In the air flow meter  10 , a direction in which the two measurement outlets  33   c  are aligned is referred to as a width direction X, a direction in which the flow channel forming portion  24  and the flange portion  27  are aligned is referred to as a height direction Y, and a direction in which the passage flow channel  31  extends is referred to as a depth direction Z. The width direction X, the height direction Y, and the depth direction Z are orthogonal to each other, and the flange surface  27   a  of the flange portion  27  extends in parallel to both the width direction X and the depth direction Z. In a state in which the air flow meter  10  is attached to the intake pipe  12   a , the inflow port  33   a  faces the upstream side of the intake passage  12 , and the outflow port  33   b  and the measurement outlet  33   c  face the downstream side. In that case, it is considered that the direction in which the intake air flows in the intake passage  12  is the depth direction Z, and the inflow direction of the inflow air from the inflow port  33   a  is likely to be the same as the depth direction Z. In the air flow meter  10 , the intake air flowing in from the inflow port  33   a  passes through the passage flow channel  31  and the measurement flow channel  32 , and flows out from the outflow port  33   b  and each measurement outlet  33   c.    
     In a flow channel boundary portion  34 , which is a boundary between the passage flow channel  31  and the measurement flow channel  32 , an intermediate portion of the passage flow channel  31  is opened toward the flange portion  27  in the height direction Y. In the flow channel boundary portion  34 , the intermediate portion of the passage flow channel  31  and the upstream end portion of the measurement flow channel  32  are connected to each other, and the upstream end portion of the measurement flow channel  32  can also be referred to as a measurement inlet. The measurement flow channel  32  has a portion extending in the depth direction Z between the flow channel boundary portion  34  and the measurement outlet  33   c , and the flow rate detection unit  22  is disposed in that portion. 
     In the air flow meter  10 , it is assumed that dust such as sand and dust enters from the inflow port  33   a  as foreign matter together with intake air. In that case, it is considered that most of the foreign matter travels in the depth direction Z along a flow of the intake air to exit from the outflow port  33   b , but some of the foreign matter enters the measurement flow channel  32  together with some of the intake air. In particular, it is considered that a large foreign matter such as a foreign matter having a relatively large mass or a foreign matter having a relatively large size tends to move linearly regardless of the flow direction of the intake air. For that reason, there is a concern that the large foreign matter collides with an inner peripheral surface  31   a  of the passage flow channel  31  and rebounds, and when a traveling direction of the large foreign matter changes, the large foreign matter more easily enters the measurement flow channel  32 . 
     On the other hand, in the present embodiment, the large foreign matter that has rebounded on the inner peripheral surface  31   a  of the passage flow channel  31  is inhibited from entering the measurement flow channel  32 . It is considered that a small foreign matter, such as a foreign matter having a relatively small mass or a foreign matter having a relatively small size, tends to change the traveling direction of the small foreign matter in accordance with the flow of intake air, and tends to bend before colliding with the inner peripheral surface  31   a  of the passage flow channel  31 . 
     As shown in  FIGS. 1 and 3 , the inner peripheral surface  31   a  of the passage flow channel  31  has a ceiling surface  36 , a bottom surface  37 , and a pair of wall surfaces  38 . The pair of wall surfaces  38  are a pair of facing surfaces facing each other across the flow channel boundary portion  34 , the inflow port  33   a , and the outflow port  33   b  in the width direction X, and the ceiling surface  36  and the bottom surface  37  are a pair of facing surfaces facing each other across the wall surfaces  38 . In the passage flow channel  31 , a portion of the ceiling surface  36  is opened, and an upstream end portion of the measurement flow channel  32  is connected to the opened portion, thereby providing the flow channel boundary portion  34 . The ceiling surface  36  has an inflow ceiling surface portion  36   a  between the inflow port  33   a  and the flow channel boundary portion  34 , and an outflow ceiling surface portion  36   b  between the flow channel boundary portion  34  and the outflow port  33   b.    
     In this example, the flow channel boundary portion  34  has an upstream boundary portion  34   a  located at the most upstream side and a downstream boundary portion  34   b  located at the most downstream side, and in the height direction Y, the upstream boundary portion  34   a  is located at a position spaced apart from the flange portion  27  from the downstream boundary portion  34   b . In that case, the upstream end portion of the measurement flow channel  32  is opened not toward the inflow port  33   a  but toward the outflow port  33   b . For that reason, even if the foreign matter traveling linearly in the depth direction Z enters from the inflow port  33   a , the foreign matter does not easily enter the measurement flow channel  32  as it is. In the above configuration, for example, even if a person looks into the passage flow channel  31  from the inflow port  33   a  in the depth direction Z, an upstream end portion of the measurement flow channel  32  cannot be visualized. 
     In the ceiling surface  36 , since the inflow ceiling surface portion  36   a  and the outflow ceiling surface portion  36   b  have step surfaces  41   a  and  41   b  and connection surfaces  42   a  and  42   b , respectively, a step facing the inflow port  33   a  is defined. In the inflow ceiling surface portion  36   a , the multiple inflow step surfaces  41   a  are disposed at depth intervals Da along the aligned direction of the inflow port  33   a  and the flow channel boundary portion  34 . In the outflow ceiling surface portion  36   b , the multiple outflow step surfaces  41   b  are aligned at depth intervals Db along the alignment direction of the flow channel boundary portion  34  and the outflow port  33   b , and the depth interval Db is smaller than a depth interval Da. The step surfaces  41   a  and  41   b  extend toward the bottom surface  37  on the ceiling surface  36 , and thus face the inflow port  33   a , and extend over the pair of wall surfaces  38 . Each inflow step surface  41   a  and each outflow step surface  41   b  extend in the same direction, specifically, both extend in a direction orthogonal to the depth direction Z. 
     The inflow connection surfaces  42   a  connect the downstream-side end portion of the upstream side inflow step surface  41   a  and the upstream end portion of the downstream side inflow step surface  41   a  in the adjacent inflow step surfaces  41   a  at the inflow ceiling surface portion  36   a , and the multiple inflow connection surfaces  42   a  are provided according to the number of the inflow step surface  41   a . The outflow connection surfaces  42   b  connect the downstream-side end portion of the upstream side outflow step surface  41   b  and the upstream end portion of the downstream side outflow step surface  41   b  in the adjacent outflow step surfaces  41   b  at the outflow ceiling surface portion  36   b , and the multiple outflow connection surfaces  42   b  are provided according to the number of the outflow step surface  41   b . The connection surfaces  42   a  and  42   b  extend in the same direction, specifically, in a direction orthogonal to the height direction Y. In other words, each inflow connection surface  42   a  is orthogonal to the inflow step surface  41   a , and each outflow connection surface  42   b  is orthogonal to the outflow step surface  41   b . In that case, in the depth direction Z, the depth dimensions of the connection surfaces  42   a  and  42   b  are the same as the depth intervals Da and Db of the adjacent step surfaces  41   a  and  41   b.    
     The inflow ceiling surface portion  36   a  and the outflow ceiling surface portion  36   b  are formed in a staircase shape as a whole by the step surfaces  41   a  and  41   b  and the connection surfaces  42   a  and  42   a . In the inflow ceiling surface portion  36   a , the step gradually increases toward the downstream side. Specifically, while the depth interval Da is uniform in each step, a height dimension Ha of the inflow step surface  41   a  along the height direction Y gradually increases as a distance from the inflow port  33   a  increases. In the step close to the inflow port  33   a , the height dimension Ha is smaller than the depth interval Da, but a difference between the height dimension Ha and the depth interval Da gradually decreases as the step comes closer to the flow channel boundary portion  34 , and in the step close to the flow channel boundary portion  34 , the height dimension Ha and the depth interval Da have substantially the same value. A height dimension Ha may be smaller than the depth interval Da. 
     On the other hand, in the outflow ceiling surface portion  36   b , the step becomes gradually smaller toward the downstream side. Specifically, while the depth interval Db is uniform in each step, a height dimension Hb of the outflow step surface  41   b  in the height direction Y gradually decreases as the step comes closer to the outflow port  33   b . In the step close to the flow channel boundary portion  34 , the height dimension Hb is larger than the depth interval Db, but the difference between the height dimension Hb and the depth interval Db gradually decreases as the step comes closer to the flow outflow port  33   b , and in the step closer to the flow outflow port  33   b , the height dimension Hb is larger than the depth interval Db. 
     In the ceiling surface  36 , the overall inclination angle of the outflow ceiling surface portion  36   b  with respect to the depth direction Z is larger than the overall inclination angle of the inflow ceiling surface portion  36   a  with respect to the depth direction Z. In this example, in a positional relationship between the upstream end portion and the downstream end portion of the inflow ceiling surface portion  36   a , a separation distance in the height direction Y is referred to as a height distance Hay, and a separation distance in the depth direction Z is referred to as a depth distance Daz. In a positional relationship between the upstream end portion and the downstream end portion of the inflow ceiling surface portion  36   a , a separation distance in the height direction Y is referred to as a height distance Hby, and a separation distance in the depth direction Z is referred to as a depth distance Daz. In that instance, a value of Hby/Dbz indicating the degree of inclination of the outflow ceiling surface portion  36   b  is larger than a value of Hay/Daz indicating the degree of inclination of the inflow ceiling surface portion  36   a . As a result, the intake air easily flows in from the inflow port  33   a , and a flow rate of the intake air in the measurement flow channel  32  easily increases. 
     The inflow ceiling surface portion  36   a  is curved so that an intermediate portion in the width direction X swells toward the flange portion  27  in accordance with the shape of the inflow port  33   a . In that case, both the upstream end portion and the downstream-side end portion of the inflow step surface  41   a  are curved. The inflow connection surfaces  42   a  are curved so as to connect the adjacent inflow step surfaces  41   a  to each other. On the other hand, the outflow port  33   b  has a substantially rectangular shape, and the outflow ceiling surface portion  36   b  is not curved. 
     According to the present embodiment described so far, since the inflow ceiling surface portion  36   a  has the inflow step surfaces  41   a , a foreign matter that has entered from the inflow port  33   a  hardly enters the measurement flow channel  32 . For example, as indicated by a solid line in  FIG. 3 , when a large foreign matter F1 entering from the inflow port  33   a  travels linearly in the depth direction Z and collides with the inflow step surface  41   a  of the inflow ceiling surface portion  36   a , there is a high possibility that the large foreign matter F1 returns back to the inflow port  33   a  so as to follow its own trajectory. As described above, the large foreign matter F1 does not easily advance to the downstream side by colliding with the inflow step surface  41   a  of the inflow ceiling surface portion  36   a  in the passage flow channel  31 , and does not easily enter the measurement flow channel  32 . 
     On the other hand, unlike the present embodiment, for example, in a configuration in which the inflow ceiling surface portion  36   a  does not have the inflow step surface  41   a  as shown in  FIG. 4 , the inflow ceiling surface portion  36   a  is not orthogonal to the depth direction Z. For that reason, it is conceivable that the large foreign matter F1 collides with the inflow ceiling surface portion  36   a  which is inclined as a whole, and advances to the downstream side while changing the traveling direction. In that case, there is a concern that the large foreign matter F1 can easily enter the measurement flow channel  32  by advancing downstream while the large foreign matter F1 rebounds at the bottom surface  37  following the inflow ceiling surface portion  36   a , as indicated by a solid line in  FIG. 4 , depending on an angle at which the large foreign matter F1 rebounds at the inflow ceiling portion  36   a . As described above, when the traveling direction of the large foreign matter F1 changes in the height direction Y with the rebound at the inflow ceiling surface portion  36   a , the possibility that the large foreign matter F1 flows into the measurement flow channel  32  is likely to increase. In that regard, according to the present embodiment, since a configuration is realized in which the traveling direction of the large foreign matter F1 rebounded at the inflow step surface  41   a  of the inflow ceiling surface portion  36   a  does not easily change in the height direction Y, the large foreign matter F1 can be inhibited from easily entering the measurement flow channel  32 . 
     In the air flow meter  10  in which the flow rate detection unit  22  is provided in the measurement flow channel  32 , if the flow rate of the intake air flowing through the measurement flow channel  32  is too small, there is a concern that the detection accuracy of the flow rate detection unit  22  is lowered. On the other hand, according to the present embodiment, since the multiple inflow step surfaces  41   a  are aligned in the depth direction Z, the cross-sectional area of the passage flow channel  31  can be gradually reduced toward the flow channel boundary portion  34  while increasing the open area of the inflow port  33   a  as much as possible. For that reason, the inflow of the intake air into the measurement flow channel  32  can be inhibited from being insufficient while inhibiting the inflow of the large foreign matter into the measurement flow channel  32  by the inflow step surface  41   a.    
     According to the present embodiment, since the multiple inflow step surfaces  41   a  are included in the inflow ceiling surface portion  36   a , the inflow ceiling surface portion  36   a  can be gradually spaced apart from the flange portion  27  as the inflow ceiling surface portion  36   a  comes closer to the flow channel boundary portion  34  from the inflow port  33   a . In that case, for example, as shown in  FIG. 3 , since the intake air G flowing in from the inflow port  33   a  tends to gradually move away from the flow channel boundary portion  34  in the Y-direction, the small foreign matter that is easily susceptible to the flow of the intake air G can be inhibited from flowing into the measurement flow channel  32  in addition to the large foreign matter. 
     According to the present embodiment, since the height dimension Ha of the passage flow channel  31  increases more as the inflow step surface  41   a  is closer to the flow channel boundary portion  34 , the change rate in the traveling direction of the intake air flowing in from the inflow port  33   a  can be gradually increased. In that case, as compared with the case where the change rate in the traveling direction of the intake air is abruptly increased, the flow of the intake air is hardly disturbed by the generation of a vortex or the like. For that reason, the flow rate of the intake air in the measurement flow channel  32  can be inhibited from being insufficient due to the intake air hardly flowing into the measurement flow channel  32  due to the turbulence of the flow, or the foreign matter entrained by the turbulence of the flow can be inhibited from entering the measurement flow channel  32 . 
     According to the present embodiment, the inflow connection surface  42   a  extends in parallel with the depth direction Z. For that reason, the inflow connection surface  42   a  can be inhibited from becoming an obstacle to the foreign matter when the foreign matter entering from the inflow port  33   a  and traveling linearly in the depth direction Z reaches the inflow step surface  41   a.    
     According to the present embodiment, since the inflow step surface  41   a  extends in parallel with the height direction Y, the inflow step surface  41   a  is orthogonal to the depth direction Z in which the intake air easily enters from the inflow port  33   a . This makes it possible to inhibit that the foreign matter, which collides with the inflow step surface  41   a  and rebounds, advances to the downstream side in the direction inclined with respect to the height direction Y, collides with the bottom surface  37 , and rebounds to enter the measurement flow channel  32 . 
     According to the present embodiment, since the outflow ceiling surface portion  36   b  has the outflow step surface  41   b , the foreign matter that has entered from the inflow port  33   a  and passed through the flow channel boundary portion  34  hardly enters the measurement flow channel  32 . For example, as indicated by a dashed line in  FIG. 3 , when a large foreign matter F2 entering from the inflow port  33   a  travels linearly in the depth direction Z and collides with the outflow step surface  41   b  of the outflow ceiling surface portion  36   b , the possibility that the large foreign matter F2 returns to the inflow port  33   a  so as to follow its own trajectory is high. As described above, the large foreign matter F2 collides with the outflow step surface  41   b  of the outflow ceiling surface portion  36   b  in the passage flow channel  31 , and thus passes through the measurement flow channel  32  which has passed once in the opposite direction, but easily travels toward the upstream side at an angle which makes it difficult to enter the measurement flow channel  32 . 
     On the contrary, unlike the present embodiment, for example, in a configuration in which the outflow ceiling surface portion  36   b  does not have the outflow step surface  41   b  as shown in  FIG. 4 , the outflow ceiling surface portion  36   b  is not orthogonal to the depth direction Z. For that reason, it is conceivable that the large foreign matter F2 collides with the outflow ceiling surface portion  36   b  which is inclined as a whole, and enters the measurement flow channel  32  by changing the traveling direction. Specifically, there is a concern that depending on the angle at which the large foreign matter F2 rebounds at the outflow ceiling surface portion  36   b , as indicated by a dashed line in  FIG. 4 , the large foreign matter F2 is advanced upstream while rebounding at the bottom surface  37  following the outflow ceiling portion  36   b , thereby making it easier to enter the measurement flow channel  32 . As described above, when the traveling direction of the large foreign matter F2 changes in the height direction Y with the rebound at the outflow ceiling surface portion  36   b , the possibility that the large foreign matter F2 flows into the measurement flow channel  32  is likely to increase. In that regard, in the present embodiment, since a configuration is realized in which the traveling direction of the large foreign matter F2 rebounded by the outflow step surface  41   b  of the outflow ceiling surface portion  36   b  is less likely to change in the height direction Y, the large foreign matter F2 can be inhibited from being likely to flow into the measurement flow channel  32 . 
     As described above, there is a concern that the detection accuracy of the flow rate detection unit  22  is lowered if the flow velocity of the intake air flowing through the measurement flow channel  32  is too small in the air flow meter  10 . On the contrary, according to the present embodiment, the cross-sectional area of the passage flow channel  31  is reduced by the outflow step surface  41   b  on the downstream side of the flow channel boundary portion  34 , thereby narrowing the passage flow channel  31 . In that case, the pressure of the intake air in the passage flow channel  31  becomes moderately high, so that the intake air is likely to flow into the measurement flow channel  32 , and the flow rate of the intake air in the measurement flow channel  32  becomes moderately large. For that reason, the deterioration of the detection accuracy of the flow rate detection unit  22  can be inhibited by the outflow step surface  41   b.    
     According to the present embodiment, since the multiple outflow step surfaces  41   b  are included in the outflow ceiling surface portion  36   b , the degree of throttling of the passage flow channel  31  can gradually increase toward the outflow port  33   b  on the downstream side of the flow channel boundary portion  34 . In that case, as compared with the configuration in which the degree of throttling of the passage flow channel  31  increases rapidly toward the outflow port  33   b , the flow of intake air is less likely to be disturbed due to generation of a vortex or the like. For that reason, the foreign matter caught in the flow disturbance can be inhibited from entering the measurement flow channel  32 . 
     According to the present embodiment, the height dimension Hb is smaller as the outflow step surface  41   b  is closer to the outflow port  33   b . For that reason, a region around the flow channel boundary portion  34  in the passage flow channel  31  can be set to be as large as possible in the height direction Y. This makes it possible to realize a configuration in which the passage flow channel  31  is gradually narrowed by the outflow step surface  41   b  toward the outflow port  33   b  while creating a situation in which the intake air easily flows into the measurement flow channel  32  from the passage  31 . 
     According to the present embodiment, the outflow connection surface  42   b  extends in parallel with the depth direction Z. For that reason, the outflow connection surface  42   b  can be inhibited from becoming an obstacle to the foreign matter when the foreign matter that has passed through the flow channel boundary portion  34  and has traveled linearly in the depth direction Z toward the outflow port  33   b  reaches the outflow step surface  41   b.    
     According to the present embodiment, since the outflow step surface  41   b  extends in parallel with the height direction Y, the outflow step surface  41   b  is orthogonal to the depth direction Z which is likely to be the traveling direction of the intake air from the inflow port  33   a . This makes it possible to inhibit that the foreign matter, which collides with the outflow step surface  41   b  and rebounds, flows back to the upstream side in the direction inclined with respect to the height direction Y, collides with the bottom surface  37 , and rebounds to enter the measurement flow channel  32 . 
     The first embodiment can be applied to various embodiments and combinations without departing from the spirit of the present disclosure. 
     As a modification A1, in the inflow step surface  41   a , only one of the upstream end portion and the downstream end portion may be curved in accordance with the shape of the inflow port  33   a , or both may not be curved. The inflow ceiling surface portion  36   a  may or may not be curved regardless of the shape of the inflow port  33   a . For example, when the inflow port  33   a  has a rectangular shape, the inflow step surface  41   a  and the inflow connection surface  42   a  may be curved. 
     In a modification A2, the outflow port  33   b  may not be formed in a rectangular shape. In that case, the outflow step surface  41   b  and the outflow connection surface  42   b  may be curved outward or inward in accordance with the shape of the outflow port  33   b.    
     Second Embodiment 
     In an air flow meter  10  of the first embodiment, the passage flow channel  31  and the measurement flow channel  32  do not overlap with each other in the width direction X, but in an air flow meter of a second embodiment, a passage flow channel and a measurement flow channel overlap with each other in the width direction X. In the second embodiment, differences from the first embodiment will be mainly described. 
     An air flow meter  50  illustrated in  FIGS. 5 to 8  is a physical quantity detection device that detects a physical quantity of an intake air in an intake passage  12  in a state of being attached to an intake pipe  12   b , as with the air flow meter  10  of the first embodiment. The air flow meter  50  includes a housing  51  and a flow rate detection unit  52 , and the housing  51  includes a flow channel forming portion  54 , an O-ring  56 , a flange portion  57 , a flange surface  57   a , and a connector portion  58 . Those members and parts correspond to members and parts having the same names as those of the first embodiment. 
     The O-ring  56  of the present embodiment does not enter an inner peripheral side of a flange portion  12   c , but is sandwiched between a tip portion of the flange portion  12   c  and the flange portion  57 . In that case, the flange surface  57   a  faces the tip end face of the flange portion  12   c  through the O-ring  56 . 
     In the housing  51 , a flow channel forming portion  54  is provided by a housing main body  51   a , a front cover  51   b , and a back cover  51   c . The housing main body  51   a  extends from the flange portion  57  in the height direction Y, and the front cover  51   b  and the back cover  51   c  are attached to the housing main body  51   a  in a state in which the front cover  51   b  and the back cover  51   c  oppose each other in parallel with each other across the housing main body  51   a  in the width direction X. Both the housing main body  51   a  and the flange portion  57  are integrally formed by molding a synthetic resin material or the like. The front cover  51   b  and the back cover  51   c  are also made of a synthetic resin material. 
     The flow channel forming portion  54  has a passage flow channel  61  and a measurement flow channel  62 , and the passage flow channel  61  has an inflow port  63   a , an outflow port  63   b , a measurement outlet  63   c , a flow channel boundary portion  64 , an upstream boundary portion  64   a , and a downstream boundary portion  64   b . An inner peripheral surface  61   a  of the passage flow channel  61  has a passage ceiling surface  66 , an inflow ceiling surface portion  66   a , an outflow ceiling surface portion  66   b , a passage bottom surface  67 , a passage wall surface  68 , an inflow step surface  71   a , and an inflow connection surface  72   a . Those members and parts correspond to members and parts having the same names as those of the first embodiment. In the present embodiment, the passage bottom surface  67  extends in parallel with the depth direction Z. 
     In the present embodiment, unlike the first embodiment, the inner peripheral surface  61   a  of the passage flow channel  61  does not have an outflow step surface and an outflow connection surface. The inflow port  63   a  is formed in a rectangular shape, and the inflow ceiling surface portion  66   a  is not curved. For that reason, both a tip portion and a base end portion of the inflow step surface  71   a  linearly extend in the width direction X. The inflow connection surface  72   a  also extends linearly in the width direction X. 
     In the present embodiment, unlike the first embodiment, the flow channel boundary portion  34  extends parallel to the depth direction Z. Even in that case, since the upstream end portion of the measurement flow channel  62  is not opened toward the side of the inflow port  63   a , even if the foreign matter traveling linearly in the depth direction Z enters from the inflow port  63   a , the foreign matter does not easily enter the measurement flow channel  62  as it is. 
     In the inflow ceiling surface portion  66   a , unlike the first embodiment, the step is neither large nor small toward the downstream side, as shown in  FIG. 9 . Specifically, a depth interval Da and a height dimension Ha of each step have the same value. In that case, the entire angle of inclination in the inflow ceiling surface portion  66   a  is the same in a portion closer to the inflow port  63   a  and a portion closer to the flow channel boundary portion  64 . 
     Returning to the description of  FIGS. 5 to 7 , the flow channel forming portion  54  has a sub-flow channel  75  in addition to the passage flow channel  61  and the measurement flow channel  62 . The sub-flow channel  75  is provided between the flange portion  57  and the measurement flow channel  62  in the height direction Y, and extends in the depth direction Z. When an upstream end portion of the sub-flow channel  75  is referred to as a sub-inlet  75   a  and a downstream end portion of the sub-flow channel  75  is referred to as a sub-outlet  75   b , the sub-inlet  75   a  is disposed between the flange portion  57  and the inflow port  33   a  in the height direction Y, and the sub-outlet  75   b  is disposed between the flange portion  57  and the outflow port  33   b . The air flow meter  50  includes a pressure detection unit  76 , a humidity detection unit  77 , and a temperature detection unit  78  in addition to the flow rate detection unit  52 , and the pressure detection unit  76  and the humidity detection unit  77  detect a pressure and a humidity of the intake air in the sub-flow channel  75 . 
     In the housing main body  51   a , a circuit board  81  is integrally provided by insert molding when the housing main body  51   a  is molded. The circuit board  81  is provided with at least one detection element for detecting a physical quantity of the intake air flowing through the intake passage  12 , and a circuit unit for processing a signal detected by the detection element. The detection element is provided at a position of the front surface or the back surface of the circuit board  81  which is exposed to the intake air, that is, at a portion which is exposed to the intake air in the intake passage  12 , the measurement flow channel  62 , and the sub-flow channel  75  and comes into contact with the intake air. The electrical connection portion between the circuit board  81  and the detection element is sealed with a synthetic resin material. The circuit unit is disposed in a circuit chamber Rc sealed by the front cover  51   b.    
     The housing main body  51   a  is provided with a groove opened toward one side or the other side in the width direction X, and a hole penetrating through the housing main body  51   a  in the width direction X. The groove and the hole are covered with the front cover  51   b  and the back cover  51   c , to thereby provide the passage flow channel  61 , the measurement flow channel  62 , and the sub-flow channel  75 . A sensor chamber Rs is provided at an intermediate position of the sub-flow channel  75 , and the sensor chamber Rs is provided with a pressure detection unit  76  and a humidity detection unit  77  as detection elements provided on the back surface of the circuit board  81 . The pressure detection unit  76  and the humidity detection unit  77  can detect a pressure and a humidity of the intake air flowing through the sub-flow channel  75 , respectively. 
     The circuit board  81  is provided at an intermediate position of the housing main body  51   a  in the width direction X in a state orthogonal to the width direction X, thereby partitioning the circuit chamber Rc and the sensor chamber Rs. The circuit chamber Rc is provided between the front cover  51   b  and the circuit board  81 , and the sensor chamber Rs is provided between the back cover  51   c  and the circuit board  81 . The circuit chamber Rc is sealed by attaching the front cover  51   b  to the housing  51 , and is completely isolated from an outside. 
     The flow channel forming portion  54  has a partition wall  84  that separates the measurement flow channel  62  and the sub-flow channel  75  from each other in the height direction Y. The circuit board  81  penetrates the partition wall  84  in the height direction Y and protrudes into the measurement flow channel  62 , and the flow rate detection unit  52  is provided in a measurement board portion  81   a  which is a protruding portion. 
     In a state in which the air flow meter  50  is attached to the intake pipe  12   a , an intermediate position between the inflow port  63   a  and the sub-inlet port  75   a  in the height direction Y is disposed at a position overlapping with or close to a center line of the intake pipe  12   a . In the above configuration, a gas at a portion close to an inner wall surface of the intake passage  12  but at a portion close to the center away from the inner wall is likely to flow into the passage flow channel  61  or the sub-flow channel  75 . In that case, the air flow meter  50  can measure the physical quantity of the gas in a portion away from the inner wall surface of the intake passage  12 , and can reduce a measurement error of the physical quantity related to a heat and a flow rate decrease in the vicinity of the inner wall surface. 
     The flow channel forming portion  54  has an inflow restriction portion  85  that restricts the inflow of the intake air from the inflow port  63   a . The inflow restriction portion  85  is a projection portion protruding from the passage bottom surface  67  of the passage flow channel  61  toward the passage ceiling surface  66 . The inflow restriction portion  85  has a downstream side surface  85   a  facing the downstream side and an upper surface  85   b  facing the passage ceiling surface  66  (hereinafter, also referred to as a ceiling side) and the downstream side surface  85   a  and the upper surface  85   b  are included in the passage bottom surface  67 . The inflow restriction portion  85  is provided in the inflow port  63   a , and the upstream end portion of the upper surface  85   b  is included in the inflow port  63   a . The downstream side surface  85   a  extends obliquely upward toward the upstream side, and the upper surface  85   b  extends parallel to the depth direction Z. 
     The inflow restriction portion  85  extends over the pair of passage wall surfaces  68 , and the opening area of the inflow port  63   a  is reduced by reducing the height dimension of the inflow port  63   a  in the height direction. The inflow restriction portion  85  is inclined with respect to the height Y by extending in a direction away from the outflow port  63   b  toward the passage ceiling surface  66  rather than extending parallel to the height direction Y. 
     In the present embodiment, as described above, the portion closer to the outflow port  63   b  in the passage flow channel  61  and the portion closer to the measurement outlet  63   c  in the measurement flow channel  62  overlap with each other in the width direction X. In the flow channel forming portion  54 , a groove is provided in the housing main body  51   a , so that the passage flow channel  61  is provided between the housing main body  51   a  and the back cover  51   c . The measurement flow channel  62  has an upstream measurement path  91 , an intermediate measurement path  92 , and a downstream measurement path  93 . The upstream measurement path  91  extends from the flow channel boundary portion  64  to the downstream side of the measurement flow channel  62  and is provided between the housing main body  51   a  and the back cover  51   c  as well as the passage flow channel  61 . The downstream measurement path  93  extends from the measurement outlet  63   c  to the upstream side of the measurement flow channel  62 , and is provided between the housing main body  51   a  and the front cover  51   b . The downstream measurement path  93  is disposed on the opposite side of the upstream measurement path  91  and the passage flow channel  61  across the housing main body  51   a  in the width direction X. 
     The intermediate measurement path  92  is a portion connecting the upstream measurement path  91  and the downstream measurement path  93  in the measurement flow channel  62 , and is disposed in a portion where a hole is provided in the housing main body  51   a , so that the intermediate measurement path  92  is provided between the front cover  51   b  and the back cover  51   c  through the hole. The intermediate measurement path  92  extends in the depth direction Z, and the intake air flows in the intermediate measurement path  92  in the opposite direction to the intake passage  12 . The intermediate measurement path  92  is divided from the sub-flow channel  75  by the partition wall  84 , and the measurement board portion  81   a  of the circuit board  81  is disposed in the intermediate measurement path  92 . For that reason, the flow rate detection unit  52  provided in the intermediate measurement path  92  detects the flow rate of the intake air flowing through the intermediate measurement path  92 . 
     In the width direction X, a width dimension of the intermediate measurement path  92  is larger than the width dimensions of the upstream measurement path  91  and the downstream measurement path  93 . The upstream measurement path  91  has a width increasing portion  91   a  whose width dimension gradually increases toward the intermediate measurement path  92 , and the downstream measurement path  93  has a width decreasing portion  93   a  whose width dimension gradually decreases away from the intermediate measurement path  92 . The housing main body  51   a  has a width increasing surface  94  forming the width increasing portion  91   a  and a width decreasing surface  95  forming the width decreasing portion  93   a . The width increasing surface  94  is included in a surface of the housing main body  51   a  facing the back cover  51   c , is not orthogonal to the width direction X, and is inclined with respect to the width direction X by facing the intermediate measurement path  92 . The width decreasing surface  95  is included in a surface of the housing main body  51   a  facing the front cover  51   b , and is inclined with respect to the width direction X by facing the intermediate measurement path  92 , similarly to the width increasing surface  94 . 
     The flow rate detection unit  52  is disposed on a surface of the measurement board portion  81   a  facing the front cover  51   b . In the intermediate measurement path  92 , the flow rate detection unit  52  is disposed on the downstream side of the width increasing surface  94 . In that case, since the flow rate detection unit  52  is hidden behind the width increasing surface  94 , even if the foreign matter enters the measurement flow channel  62  from the passage flow channel  61 , the width increasing surface  94  becomes an obstacle and the foreign matter is less likely to reach the flow rate detection unit  52 . 
     According to the present embodiment described so far, similarly to the first embodiment, since the inflow ceiling surface portion  66   a  has the inflow step surface  71   a , the foreign matter that has entered from the inflow port  63   a  is less likely to enter the measurement flow channel  62 . The inflow step surface  71   a  is orthogonal to the depth direction Z. For that reason, similarly to  FIG. 3 , when the large foreign matter F1 entering from the inflow port  63   a  moves linearly in the depth direction Z and collides with the inflow step surface  71   a  as shown in  FIG. 8 , it is considered that the possibility that the large foreign matter F1 returns to the inflow port  63   a  so as to follow its own trajectory is high. On the other hand, unlike the present embodiment, in the configuration in which the inflow ceiling surface portion  66   a  does not have the inflow step surface  71   a  as shown in  FIG. 10 , the inflow ceiling surface portion  66   a  is not orthogonal to the depth direction Z, similarly to  FIG. 4 . For that reason, there is a concern that the large foreign matter F1 collides with the inflow ceiling surface portion  66   a  which is inclined as a whole, and enters the measurement flow channel  62  while changing the traveling direction. In that regard, in the present embodiment, the inflow ceiling surface portion  66   a  restricts the rebound direction of the large foreign matter F1, thereby being capable of inhibiting the large foreign matter F1 from entering the measurement flow channel  62 . 
     According to the present embodiment, since the inflow restriction portion  85  is provided on the passage bottom surface  67  on the opposite side of the inflow step surface  71   a  across the inflow port  63   a , a probability that the foreign matter entering from the inflow port  63   a  and traveling linearly collides with the inflow step surface  71   a . This is because a region of the inflow port  63   a  which does not face the inflow step surface  71   a , that is, a region which is not aligned with the inflow step surface  71   a  in the depth direction Z can be closed by the inflow step surface  71   a . For that reason, the foreign matter can be inhibited from entering the measurement flow channel  62  without colliding with the inflow step surface  71   a.    
     The second embodiment can be applied to various embodiments and combinations without departing from the spirit of the present disclosure. 
     As a modification B1, the inflow step surface  71   a  may not be parallel to the depth direction Z. For example, as shown in  FIG. 11 , the inflow step surface  71   a  extends obliquely upward toward the upstream side. In that configuration, the inflow connection surface  72   a  is orthogonal to the inflow step surface  71   a , and the inflow connection surface  72   a  extends obliquely downward toward the upstream side. When an angle between the inflow step surface  71   a  and the depth direction Z is referred to as a step angle θz, and an angle between the inflow connection surface  72   a  and the height direction Y is referred to as a connection angle θy, the step angle θz and the connection angle θy are the same angles. The angles θz and θy are positive and have relatively small absolute values of several degrees to several tens of degrees. For that reason, for example, even if the large foreign matter F1 traveling linearly in the depth direction Z collides with the inflow step surface  71   a  or the inflow connection surface  72   a , the large foreign matter F1 tends to return toward the inflow port  63   a  in substantially the same direction as the depth direction Z. 
     As shown in  FIG. 12 , the inflow step surface  71   a  extends obliquely downward toward the upstream side. In that configuration, the inflow connection surface  72   a  is orthogonal to the inflow step surface  71   a , and the inflow connection surface  72   a  extends obliquely downward toward the downstream side. In that case, the step angle θz and the connection angle θy are negative values and have relatively small absolute values of several degrees to several tens of degrees. Even in that case, the large foreign matter F1 rebounded at the inflow step surface  71   a  and the inflow connection surface  72   a  tends to return toward the inflow port  63   a  in substantially the same direction as the depth direction Z. 
     As a modification B2, the inflow step surface  71   a  and the inflow connection surface  72   a  may not be orthogonal to each other. For example, the angle between the inflow step surface  71   a  and the inflow connection surface  72   a  may be smaller than 90 degrees or larger than 90 degrees. It is preferable that a difference between the angle and 90 degrees is small to the extent that when the large foreign matter F1 traveling linearly in the depth direction Z collides with the inflow step surface  71   a  or the inflow connection surface  72   a , the large foreign matter F1 tends to return toward the inflow port  63   a  in substantially the same direction as the depth direction Z. Preferred values include relatively small absolute values, such as a few degrees to several degrees severity. 
     As a modification B3, the height dimension Ha of the inflow step surface  71   a  may not be the same in each step of the inflow ceiling surface portion  66   a . For example, as shown in  FIG. 13 , the height dimension Ha of the inflow step surface  71   a  gradually decreases as a distance from the inflow port  63   a  increases. In that configuration, the depth interval Da is the same for each step. The height dimension Ha of the inflow step surface  71   a  may gradually increase as the distance from the inflow port  63   a  increases. 
     As a modification B4, in each step of the inflow ceiling surface portion  66   a , both the height dimension Ha and the depth interval Da of the inflow step surface  71   a  may be different from each other. For example, as shown in  FIG. 14 , in each step of the inflow ceiling surface portion  66   a , both the height dimension Ha and the depth interval Da gradually increase as the distance from the inflow port  63   a  increases. It should be noted that both the height dimension Ha and the depth interval Da may gradually decrease as the distance from the inflow port  63   a  increases. 
     As a modification B5, similarly to the first embodiment, the air flow meter  50  of the second embodiment may have an outflow step surface and an outflow connection surface. For example, as shown in  FIG. 15 , in the inner peripheral surface  61   a  of the passage flow channel  61 , an outflow ceiling surface portion  66   b  of the passage ceiling surface  66  has an outflow step surface  71   b  and an outflow connection surface  72   b . While the outflow step surface  71   b  and the outflow connection surface  72   b  correspond to the same named parts of the first embodiment, in this configuration, the inflow ceiling surface portion  66   a  does not have the inflow step surface  71   a  and the inflow connection surface  72   a . Similarly, in the above configuration, the outflow step surface  71   b  is orthogonal to the depth direction. For that reason, similarly to  FIG. 3 , when the large foreign matter F2 entering from the inflow port  63   a  travels linearly in the depth direction Z and collides with the outflow step surface  71   b  as shown in  FIG. 15 , it is considered highly likely that the large foreign matter F2 returns to the inflow port  63   a  so as to follow its own trajectory. 
     On the other hand, unlike the present embodiment, in the configuration in which the outflow ceiling surface portion  66   b  does not have the outflow step surface  71   b  as illustrated in  FIG. 16 , the outflow ceiling surface portion  66   b  does not have a portion perpendicular to the depth direction as illustrated in  FIG. 4 . For that reason, there is a concern that the large foreign matter F2 collides with the outflow ceiling surface portion  66   b  which is inclined as a whole, and enters the measurement flow channel  62  by changing the traveling direction. In that regard, according to the present embodiment, the outflow ceiling surface portion  66   b  restricts the rebound direction of the large foreign matter F2, thereby being capable of inhibiting the large foreign matter F2 from entering the measurement flow channel  62 . 
     The modification B1 may be applied to the modification B4, and the outflow step surface  71   b  may not be parallel to the depth direction Zb. For example, the outflow step surface  71   b  extends obliquely upward or obliquely downward toward the upstream side. In addition, the modification B2 may be applied to the modification B4, and the outflow step surface  71   b  and the outflow connection surface  72   b  may not be orthogonal to each other. 
     As a modification B6, in the modification B5, as shown in  FIG. 17 , the passage ceiling surface  66  may have the inflow step surface  71   a  and the inflow connection surface  72   a  in addition to the outflow step surface  71   b  and the outflow connection surface  72   b . In the above configuration, similarly to the first embodiment, both of the large foreign matter F1 colliding with the inflow ceiling surface portion  66   a  and the large foreign matter F2 colliding with the outflow ceiling surface portion  66   b  can exert a deterrent force against entering the measurement flow channel  62 . 
     As a modification B7, a step may not be formed on the entire inflow ceiling surface portion  66   a . For example, as shown in  FIG. 18 , the inflow ceiling surface portion  66   a  has an inflow non-step surface  73   a  in addition to the inflow step surface  71   a  and the inflow connection surface  72   a . The inflow non-step surface  73   a  extends obliquely downward from the downstream end portion of the inflow step surface  71   a  disposed at the most downstream side toward the downstream side, and the downstream end portion of the inflow non-step surface  73   a  is disposed at the upstream boundary portion  64   a . Even in the above configuration, a deterrent force against the large foreign matter F1 entering the measurement flow channel  62  can be exerted on the inflow step surface  71   a . The inflow non-step surface  73   a  may be disposed upstream of any of the inflow step surfaces  71   a  or may be disposed between the multiple inflow step surfaces  71   a . The inflow non-step surface  73   a  may extend obliquely upward toward the downstream side, or may extend parallel to the depth direction Z. 
     The modification B7 is applied to the modification B4, and the step may not be formed on the entire outflow ceiling surface portion  66   b . For example, as shown in  FIG. 18 , the outflow ceiling surface portion  66   b  has an outflow non-step surface  73   b  in addition to the outflow step surface  71   b  and the outflow connection surface  72   b . The outflow non-step surface  73   b  extends obliquely downward from the downstream end portion of the outflow step surface  71   b  disposed at the most downstream side toward the downstream side, and a downstream end portion of the outflow non-step surface  73   b  is disposed at the outflow port  63   b . Even in the above configuration, the outflow step surface  71   b  can exert a deterrent force against the large foreign matter F2 entering the measurement flow channel  62 . The outflow non-step surface  73   b  may be disposed on the upstream side of any of the outflow step surfaces  71   b , or may be disposed between the multiple outflow step surfaces  71   b . The outflow non-step surface  73   b  may extend obliquely upward toward the downstream side, or may extend parallel to the depth direction Z. 
     As a modification B8, the passage bottom surface  67  may be inclined with respect to the depth direction Z. For example, as shown in  FIG. 19 , the passage bottom surface  67  extends obliquely upward toward the upstream side. In the above configuration, the passage bottom surface  67  inclined with respect to the depth direction Z is extended linearly over the inflow port  63   a  and the outflow port  63   b . In that case, the flow channel forming portion  54  does not have the inflow restriction portion  85 . 
     As a modification B9, as shown in  FIG. 20 , the flow channel forming portion  54  may not have the inflow restriction portion  85 . In that case, at least a part of the outflow ceiling surface portion  66   b  is not hidden from the upstream side by the inflow restriction portion  85  in the depth direction Z. For that reason, all the outflow step surfaces  71   b  are exposed to the upstream side through the inflow port  63   a.    
     As a modification B10, the passage bottom surface  67  may have steps. For example, as shown in  FIG. 21 , the passage bottom surface  67  has bottom step surfaces  67   a  and bottom connection surfaces  67   b . The multiple bottom step surfaces  67   a  are perpendicular to the depth direction Z in the same manner as the inflow step surfaces  71   a  and the outflow step surfaces  71   b , and aligned in the depth direction Z at predetermined intervals. An installation interval of the bottom step surfaces  67   a  is larger than the depth interval Da of the inflow step surfaces  71   a  and the depth interval Db of the outflow step surfaces  71   b . Like the inflow connection surfaces  72   a  and the outflow connection surfaces  72   b , the bottom connection surfaces  67   b  extend parallel to the depth direction Z and connect the adjacent bottom step surfaces  67   a.    
     In the configuration in which the passage bottom surface  67  has the bottom step surfaces  67   a  and the bottom connection surfaces  67   b , the passage ceiling surface  66  may not have the inflow step surfaces  71   a  and the outflow step surfaces  71   b . In that case, both of the large foreign matter colliding with the passage bottom surface  67  can exert a deterrent force against entering the measurement flow channel  62  by changing the traveling direction of the large foreign matter. 
     As a modification B11, the passage wall surface  68  may have steps. For example, as shown in  FIG. 22 , the passage wall surface  68  has wall step surfaces  68   a  and wall connection surfaces  68   b . The wall step surfaces  68   a  are orthogonal to the depth direction Z in the same manner as the bottom step surfaces  67   a  of the modification B10 described above and are aligned in the depth direction Z at predetermined intervals. An installation interval of the wall step surfaces  68   a  is larger than the depth interval Da of the inflow step surfaces  71   a  and the depth interval Db of the outflow step surfaces  71   b , and is, for example, the same as the installation interval of the bottom step surfaces  67   a . Specifically, the wall step surfaces  68   a  and the bottom step surfaces  67   a  are connected to each other. The wall connection surfaces  68   b  extend in parallel to the depth direction Z, similarly to the bottom connection surfaces  67   b  of the modification B10, and connect the adjacent wall step surfaces  68   a . The wall step surfaces  68   a  and the wall connection surfaces  68   b  are formed on at least one of the pair of passage wall surfaces  68 . 
     As a modification B12, the depth interval Da of the inflow step surfaces  71   a  may not be larger than the depth interval Db of the outflow step surfaces  71   b . For example, the depth interval Da may be the same as or smaller than the depth interval Db. 
     As a modification B13, the inflow step surfaces  71   a  may be provided in the inflow ceiling surface portion  66   a  and the outflow ceiling surface portion  66   b  one by one. The inflow step surfaces  71   a  may be provided on only one of the inflow ceiling surface portion  66   a  and the outflow ceiling surface portion  66   b.    
     Third Embodiment 
     An air flow meter  50  according to a third embodiment has a parallel region extending linearly in parallel with the depth direction Z. In the present embodiment, differences from the second embodiment will be mainly described. 
     As shown in  FIG. 23 , a passage flow channel  61  has a parallel region  101 , a ceiling-side region  102 , and a hidden region  103 . The parallel region  101  is a region extending linearly in the depth direction Z so as to connect an inflow port  63   a  and an outflow port  63   b , and an upstream end portion of the parallel region  101  is included in the inflow port  63   a  and a downstream end portion is included in the outflow port  63   b . The ceiling-side region  102  is a region closer to a ceiling than the parallel region  101  in the height direction Y, and extends from the inflow port  63   a  toward a downstream side. In that case, an upstream end portion of the ceiling-side region  102  is included in the inflow port  63   a . The hidden region  103  is a region located closer to a passage bottom surface  67  side (hereinafter, also referred to as a bottom side) than the parallel region  101  in the height direction Y, and extends from the outflow port  63   b  to the upstream side. In that case, a downstream end portion of the hidden region  103  is included in the outflow port  63   b . All of the regions  101  to  103  are virtual regions, and the passage flow channel  61  is not actually divided into the regions  101  to  103 . In  FIGS. 23 to 25 , the parallel region  101  is illustrated by dot hatching. 
     As shown in  FIGS. 23 and 24 , the inflow port  63   a  has a first entrance region  63   a   1  included in the parallel region  101  and a second entrance region  63   a   2  included in the ceiling-side region  102 . In the inflow port  63   a , the first entrance region  63   a   1  is disposed on a flange tip side of the second entrance region  63   a   2 , and those regions  63   a   1  and  63   a   2  are aligned in the height direction Y so as to divide the inflow port  63   a  into two. The parallel region  101  is a region in which the first entrance region  63   a   1  is projected toward the downstream side, and the projection region reaches the outflow port  63   b . On the other hand, since the ceiling-side region  102  gradually comes closer to the passage bottom surface  67  as the inflow ceiling surface portion  66   a  comes closer to a flow channel boundary portion  64 , the ceiling-side region  102  is blocked by the inflow ceiling surface portion  66   a  from extending downstream in the depth direction Z. In this case, the ceiling-side region  102  is disposed on the upstream side of the inflow ceiling surface portion  66   a.    
     As shown in  FIGS. 23 and 25 , the outflow port  63   b  has a first exit region  63   b   1  included in the parallel region  101  and a second exit region  63   b   2  included in the hidden region  103 . In the outflow port  63   b , the first exit region  63   b   1  is disposed closer to a base end side of the flange than the second exit region  63   b   2 , and the regions  63   b   1  and  63   b   2  are aligned in the height direction Y so as to divide the outflow port  63   b  into two. The parallel region  101  may be referred to as a region in which the first exit region  63   b   1  is projected toward the upstream side. On the other hand, although the hidden region  103  extends upstream along the passage bottom surface  67 , the hidden region  103  is blocked by the inflow restriction portion  85  from extending upstream in the depth direction Z due to the protrusion of the inflow restriction portion  85  from the passage bottom surface  67 . In that case, the hidden region  103  is disposed on the downstream side of the inflow restriction portion  85 , and is in a state of being hidden from the upstream side by the inflow restriction portion  85 . 
     As shown in  FIG. 23 , an inner peripheral surface  61   a  of the passage flow channel  61  has a height narrowing surface  105 . The height narrowing surface  105  is included in the passage bottom surface  67  and extends parallel to the width direction X over a pair of passage wall surfaces  68 . The height narrowing surface  105  is disposed closer to the outflow port  63   b  than the flow channel boundary portion  64  in the depth direction Z, and extends from the outflow port  63   b  toward the upstream side. The height narrowing surface  105  gradually decreases a height dimension Hc of the passage flow channel  61  as the height narrowing surface  105  comes closer to the outflow port  63   b.    
     The height narrowing surface  105  gradually comes closer to the passage ceiling surface  66  as the height narrowing surface  105  comes closer to the outflow port  63   b , and continuously restricts the passage flow channel  61 . In the width direction X, the width dimension of the passage flow channel  61  is uniform. As the height dimension Hc of the passage flow channel  61  gradually decreases toward the outflow port  63   b , and the cross-sectional area of the passage flow channel  61  also gradually decreases. In that case, both the height dimension Hc and the cross-sectional area are smallest at the outflow port  63   b  on the downstream side of the flow channel boundary portion  64  in the passage flow channel  61 . 
     The height dimension of the parallel region  101  remains constant in any part in the depth direction Z. On the other hand, the height dimension of the ceiling-side region  102  gradually decreases as the distance from the inflow port  63   a  increases. In this example, the passage bottom surface  67  has, in addition to the height narrowing surface  105 , a parallel bottom surface portion  106  extending in parallel with the depth direction Z, and the parallel bottom surface portion  106  extends from the upstream end portion of the height narrowing surface  105  toward the upstream side. In that case, the height dimension of the hidden region  103  is constant for any portion of the area where the parallel bottom surface portion  106  is located, but gradually decreases toward the outflow port  63   b  as the area where the height narrowing surface  105  is located. 
     In the inflow port  63   a , the height dimension of the parallel region  101  is smaller than the height dimension of the ceiling-side region  102 . In other words, the height dimension of the first entrance region  63   a   1  is smaller than the height dimension of the second entrance region  63   a   2 . In that case, the second entrance region  63   a   2  and the ceiling-side region  102  inhibits the insufficiency of the inflow amount of the intake air into the passage flow channel  61  while securing the parallel region  101 . In the outflow port  63   b , the height dimension of the parallel region  101  is smaller than the height dimension of the hidden region  103 . In other words, the height dimension of the first exit region  63   b   1  is larger than the height dimension of the second exit region  63   b   2 . In that case, since the parallel region  101  is secured as large as possible at the outflow port  63   b , the foreign matter traveling linearly through the parallel region  101  is easily discharged from the outflow port  63   b  as it is. 
     The height narrowing surface  105  is disposed on the downstream side of the inflow restriction portion  85  in the depth direction Z, and is hidden from the upstream side by the inflow restriction portion  85 . For that reason, in the depth direction Z, the height narrowing surface  105  is not exposed to the upstream side from the inflow port  63   a  due to the presence of the inflow restriction portion  85 . For example, when a person looks into the passage flow channel  61  through the inflow port  63   a  in the depth direction Z, the height narrowing surface  105  cannot be visually recognized because the sight line is blocked by the inflow restriction portion  85 . However, in a direction angled with respect to the depth direction Z, the height narrowing surface  105  may be exposed from the inflow port  63   a , and a person looking into the depth side of the inflow restriction portion  85  can visually recognize the height narrowing surface  105  from that direction. 
     For example, when a large foreign matter F3 traveling linearly in the depth direction Z enters the passage flow channel  61  from the first entrance region  63   a   1  of the inflow port  63   a , the large foreign matter F3 simply travels linearly in the parallel region  101  and exits from the first exit region  63   b   1  of the outflow port  63   b . For that reason, even if the passage flow channel  61  is narrowed by the height narrowing surface  105 , the large foreign matter F3 traveling linearly in the depth direction Z that travels linearly in the parallel region  101  does not easily collide with the height narrowing surface  105  or enter the measurement flow channel  62 . 
     On the other hand, unlike the present embodiment, in the configuration in which the height narrowing surface  105  is exposed to the upstream side from the inflow port  63   a  in the depth direction Z, there is a concern that foreign matter will collide with the height narrowing surface  105  and rebounds, thereby easily entering the measurement flow channel  32 . For example, as shown in  FIG. 26 , in the configuration in which the height narrowing surface  105  is exposed to the upstream side from the inflow port  63   a  in the depth direction Z with no provision of the inflow restriction portion  85 , it is assumed that a large foreign matter F4 traveling linearly in the depth direction Z collides with the height narrowing surface  105 . In that case, depending on the angle at which the large foreign matter F4 rebounds at the height narrowing surface  105 , the large foreign matter F4 may easily enter the measurement flow channel  62  while the large foreign matter F4 rebounds at the outflow ceiling surface portion  66   b  following the height narrowing surface  105  and advances toward the upstream side. As described above, when the traveling direction of the large foreign matter F4 rebounded at the height narrowing surface  105  changes with respect to the height direction Y, the possibility that the large foreign matter F4 enters the measurement flow channel  62  is likely to be increased. In that regard, in the present embodiment, since the large foreign matter F4 that has traveled linearly in the depth direction Z is configured to hardly collide with the height narrowing surface  105 , the large foreign matter F4 is inhibited from entering the measurement flow channel  62 . 
     In the present embodiment, the outflow ceiling surface portion  66   b  and the flow channel boundary portion  64  extend parallel to the depth direction Z, and the upstream end portion of the measurement flow channel  62  is opened to the flange tip side in the height direction Y. In that case, the upstream end portion of the measurement flow channel  62  is not opened toward either the inflow port  63   a  or the outflow port  63   b . The parallel region  101  extends in parallel with the flow channel boundary portion  64 , and the outflow ceiling surface portion  66   b  and the flow channel boundary portion  64  define a ceiling-side range of the parallel region  101 . Because the flow channel boundary portion  64  extends in parallel with the depth direction Z, the flow channel boundary portion  64  is not exposed to the upstream side from the inflow port  63   a . For that reason, for example, the large foreign matter F3 traveling linearly in the depth direction Z in the parallel region  101  does not easily enter the measurement flow channel  62  without changing the traveling direction. The parallel region  101  extends in parallel with the upper surface  85   b  of the inflow restriction portion  85 , and the upper surface  85   b  defines a bottom side range of the parallel region  101 . 
     According to the present embodiment described so far, since the height narrowing surface  105  is not exposed to the upstream side from the inflow port  63   a  in the depth direction Z while reserving the parallel region  101  in the passage flow channel  61 , a configuration in which the foreign matter is less likely to collide with the height narrowing surface  105  can be realized. This makes it possible to inhibit that the foreign matter entering the passage flow channel  61  through the inflow port  63   a  and traveling linearly collides with the height narrowing surface  105  and rebounds to return to the upstream side and enter the measurement flow channel  62 , despite passing through the flow channel boundary portion  64 . 
     In the passage flow channel  61 , the parallel region  101  is reserved as a projection region of the first entrance region  63   a   1  of the inflow port  63   a . For that reason, the foreign matter that travels linearly in the depth direction Z in the parallel region  101  tends to exit from the outflow port  63   b  without colliding with any portion of the inner peripheral surface  61   a  of the passage flow channel  61 . As described above, for example, as compared with the configuration in which the region extending linearly in the depth direction Z is not secured in the passage flow channel  61 , the possibility that the foreign matter collides with the inner peripheral surface  61   a  of the passage flow channel  61  is reduced, thereby being capable of reducing the entry of the foreign matter into the measurement flow channel  62 . 
     In addition, since the height narrowing surface  105  reduces the passage flow channel  61  on the downstream side of the flow channel boundary portion  64 , the amount of intake air flowing from the passage flow channel  61  into the measurement flow channel  62  is likely to increase. In this example, because the flow rate detection unit  52  is a thermal type flow rate sensor, it is preferable that a flow rate of the intake air in the measurement flow channel  62  is high to some extent in order to properly maintain the detection accuracy of the flow rate detection unit  52 . In other words, it is preferable that the flow rate of the intake air from the passage flow channel  61  to the measurement flow channel  62  is somewhat high. The inflow amount into the measurement flow channel  62  increases or decreases in accordance with a relationship between the cross-sectional area and the flow channel length of the passage flow channel  61  and the measurement flow channel  62 , and it is conceivable that the inflow amount increases as a minimum cross-sectional area in the passage flow channel  61  decreases. On the other hand, according to the present embodiment, since the minimum cross-sectional area of the passage flow channel  61  is reduced by the amount corresponding to the provision of the height narrowing surface  105 , the inflow amount into the measurement flow channel  62  is increased as compared with the configuration in which the height narrowing surface  105  is not provided. This makes it possible to optimize the detection accuracy of the flow rate detection unit  52  in the measurement flow channel  62 . 
     According to the present embodiment, since the flow channel boundary portion  64  is not exposed to the upstream side from the inflow port  63   a  in the depth direction Z, the foreign matter entering from the inflow port  63   a  can be inhibited from directly entering the measurement flow channel  62  without colliding with the inner peripheral surface  61   a  of the passage flow channel  61 . This makes it possible to exert a deterrent force against lowering of the detection accuracy of the flow rate detection unit  52  due to the foreign matter adhering to the flow rate detection unit  52  or the like. 
     According to the present embodiment, since the height narrowing surface  105  is an inclined surface, the height dimension Hc and the cross-sectional area of the passage flow channel  61  are gradually reduced. For that reason, as compared with a configuration in which the height dimension Hc and the cross-sectional area of the passage flow channel  61  are rapidly reduced, for example, turbulence of an air flow is less likely to occur in the periphery of the height narrowing surface  105 . In that case, since the turbulence is less likely to occur also in the intake air flowing into the measurement flow channel  62 , the detection accuracy of the flow rate detection unit  52  can be inhibited from being lowered due to the turbulence in the air flow generated in the measurement flow channel  62 . 
     According to the present embodiment, the passage bottom surface  67  has the parallel bottom surface portion  106  extending parallel to the parallel region  101 . In that case, for example, the flow of the intake air in the parallel region  101  is less likely to be disturbed as compared with a configuration in which the passage bottom surface  67  does not have a portion extending in parallel with the parallel region  101 . For that reason, the parallel bottom surface portion  106  can urge the foreign matter that travels linearly in the depth direction Z in the parallel region  101  to exit from the outflow port  63   b  as it is. 
     According to the present embodiment, since the inflow restriction portion  85  is simply provided so as to cover and hide the height narrowing surface  105  from the upstream side, the foreign matter traveling linearly in the depth direction Z is less likely to collide with the height narrowing surface  105 . For example, unlike the present embodiment, when the height narrowing surface  105  is hidden on the downstream side of the inflow ceiling surface portion  66   a , there is a concern that a large number of considerations such as the position of the flow channel boundary portion  64  and the branching angle of the measurement flow channel  62  with respect to the passage flow channel  61  occur in the stage of design change. On the other hand, in the method of providing the inflow restriction portion  85 , although there is a need to optimize the inflow amount from the inflow port  63   a  at the stage of design change, it is considered that a design load is relatively easily reduced. 
     According to the present embodiment, the whole of the outflow port  63   b  is not included in the parallel region  101 , but the first exit region  63   b   1  of the outflow port  63   b  is included in the parallel region  101 , while the second exit region  63   b   2  is not included in the parallel region  101 . For that reason, for example, when the turbulence of the air flow occurs in the passage flow channel  61 , the possibility that the turbulence is included not in the parallel region  101  but in the hidden region  103 , for example, can be ensured. In other words, the possibility that the turbulence or the like of the air flow is discharged to the outside from the second exit region  63   b   2  instead of the first exit region  63   b   1 . This makes it possible to inhibit a state in which the foreign matter does not easily travel linearly in the depth direction Z in the parallel region  101  due to the turbulence of the air flow or the like. 
     The third embodiment can be applied to various embodiments and combinations without departing from the spirit of the present disclosure. 
     As a modification C1, the inflow ceiling surface portion  66   a  may have steps. For example, the second embodiment is applied, and as shown in  FIG. 27 , the inflow ceiling surface portion  66   a  has an inflow step surface  71   a  and an inflow connection surface  72   a . Also in the above configuration, the ceiling-side region  102  is formed between the inflow ceiling surface portion  66   a  and the parallel region  101 . In the above configuration, when a foreign matter traveling linearly in the depth direction Z enters the ceiling-side region  102 , the foreign matter collides with the inflow step surface  71   a  and rebounds to the inflow port  63   a  side, thereby inhibiting the entry of the foreign matter into the measurement flow channel  62 . 
     As a modification example C2, the height narrowing surface  105  may have steps. For example, the modification B10 is applied, and as shown in  FIG. 28 , the height narrowing surface  105  has bottom step surfaces  67   a  and bottom connection surfaces  67   b . The height narrowing surface  105  with the above configuration does not continuously narrows the passage flow channel  61  while coming closer to the outflow port  63   b , but narrows the passage flow channel  61  in a stepwise manner. In the above configuration, the height dimension Hc and the cross-sectional area of the passage flow channel  61  are gradually reduced toward the outflow port  63   b . In this example, a downstream end portion of the bottom connection surface  67   b  disposed on the most downstream side is included in the outflow port  63   b , and the height dimension Hc and the cross-sectional area of the portion formed by the bottom connection surface  67   b  are the smallest in the passage flow channel  61 . 
     As a modification C3, both the inflow ceiling surface portion  66   a  and the height narrowing surface  105  may have steps by combining the modification C1 and the modification C2 together. For example, as shown in  FIG. 29 , the inflow ceiling surface portion  66   a  has an inflow step surface  71   a  and an inflow connection surface  72   a , and the height narrowing surface  105  has a bottom step surface  67   a  and a bottom connection surface  67   b.    
     As a modification C4, the inner peripheral surface  61   a  of the passage flow channel  61  may have multiple height narrowing surfaces. For example, as shown in  FIG. 30 , the inner peripheral surface  61   a  has a bottom restriction surface  105   a  and a ceiling restriction surface  105   b  as height narrowing surfaces. The bottom restriction surface  105   a  is the height narrowing surface  105  of the third embodiment, and is included in the passage bottom surface  67 . The ceiling restriction surface  105   b  is included in the outflow ceiling surface portion  66   b , and extends over the pair of passage wall surfaces  68  in the same manner as the bottom restriction surface  105   a . A downstream end portion of the ceiling restriction surface  105   b  is included in the outflow port  63   b , and the ceiling restriction surface  105   b  gradually comes closer to the passage bottom surface  67  as the ceiling restriction surface  105   b  comes closer to the outflow port  63   b  in the height direction Y. In addition, almost the whole of the outflow ceiling surface portion  66   b  is the ceiling restriction surface  105   b . In the above configuration, since both the bottom restriction surface  105   a  and the ceiling restriction surface  105   b  narrow the passage flow channel  61 , the degree of narrowing the passage flow channel  61  can be set to be as large as possible. 
     As a modification C5, the passage flow channel  61  may have a plurality of hidden regions. For example, as shown in  FIG. 30  and  FIG. 31 , the passage flow channel  61  is configured to have a bottom hidden region  103   a  and a ceiling hidden region  103   b  as hiding regions. The bottom hidden region  103   a  is the hidden region  103  of the third embodiment, and is formed between the parallel region  101  and the passage bottom surface  67 . The ceiling hidden region  103   b  is a region formed between the parallel region  101  and the outflow ceiling surface portion  66   b.    
     For example, as shown in  FIG. 31 , the ceiling hidden region  103   b  may extend from the outflow port  63   b  toward the downstream side. In the above configuration, the outflow port  63   b  has multiple second exit regions  63   b   2 , the bottom hidden region  103   a  extends from the second exit regions  63   b   2  on the bottom side, and the ceiling hidden region  103   b  extends from the second exit regions  63   b   2  on the ceiling-side toward the upstream side. The ceiling hidden region  103   b  is disposed on the downstream side of the inflow ceiling surface portion  66   a  in the depth direction Z, and is in a state of being hidden from the upstream side by the inflow ceiling surface portion  66   a.    
     The ceiling hidden region  103   b  may be formed independently of the outflow port  63   b , as shown in  FIG. 31 , for example. In the above configuration, as compared with the third embodiment, the downstream boundary portion  64   b  of the flow channel boundary portion  64  is disposed at a position away from the passage bottom surface  67 . In that case, the flow channel boundary portion  64  does not extend in parallel to the depth direction Z, but extends obliquely to the bottom side toward the downstream side, thereby being inclined with respect to the depth direction Z. In this example, the upstream end portion of the ceiling restriction surface  105   b  is included in the outflow port  63   b . As a result, the passage flow channel  61  is shaped such that a portion around the downstream boundary portion  64   b  expands toward a side opposite to the passage bottom surface  67 , and that portion is the ceiling hidden region  103   b . The ceiling hidden region  103   b  is a region surrounded by the ceiling restriction surface  105   b , the flow channel boundary portion  64 , and the parallel region  101 . 
     As a modification C6, the inner peripheral surface  61   a  of the passage flow channel  61  may have a width narrowing surface that narrows the passage flow channel  61  in the width direction as a distance from the outflow port  63   b  decreases. Specifically, at least one of the pair of passage wall surfaces  68  may include the width narrowing surface. For example, as shown in  FIG. 32 , one of the pair of passage wall surfaces  68  includes a width narrowing surface  107 . The width narrowing surface  107  extends in parallel to the height direction Y in a state where the width narrowing surface  107  extends over the outflow ceiling surface portion  66   b  and the passage bottom surface  67 . The width narrowing surface  107  is disposed closer to the outflow port  63   b  than the flow channel boundary portion  64  in the depth direction Z, and extends from the outflow port  63   b  toward the upstream side. The width narrowing surface  107  gradually decreases the width dimension Wa of the passage flow channel  61  as the width narrowing surface  107  comes closer to the outflow port  63   b . The width narrowing surface  107  comes gradually closer to the other passage wall surface  68  as the width narrowing surface  107  comes closer to the outflow port  63   b , and continuously reduces the width dimension Wa and the cross-sectional area of the passage flow channel  61 . 
     The parallel region  101  is a region between the width narrowing surface  107  and the passage wall surface  68  without the width narrowing surface  107  in the width direction X. The passage flow channel  61  has, in addition to the parallel region  101 , a side region  104  provided on the side of the parallel region  101  in the width direction X. The side region  104  is a region extending from the inflow port  63   a  toward the downstream side, and is disposed on the upstream side of the width narrowing surface  107 . In the above configuration, it is considered that the foreign matter that has entered the side region  104  from the inflow port  63   a  travels linearly in the depth direction Z to rebound at the width narrowing surface  107 , but it is considered that the traveling direction of the foreign matter is likely to change in the width direction X in the rebound, but is less likely to change in the height direction Y. For that reason, it is difficult for the foreign matter to easily enter the measurement flow channel  62  due to collision with the width narrowing surface  107 . 
     The passage wall surface  68  having the width narrowing surface  107  has a parallel wall surface portion  108  extending parallel to the depth direction Z. The parallel wall surface portion  108  extends from the upstream end portion of the width narrowing surface  107  toward the upstream side, and the upstream end portion of the parallel wall surface portion  108  is included in the inflow port  63   a . As a result, the parallel wall surface portion  108  prompts the foreign matter that travels linearly in the depth direction Z in the parallel region  101  to move linearly as it is and exit from the outflow port  63   b.    
     The width narrowing surface  107  may have steps instead of an inclined surface. For example, similarly to the passage ceiling surface  66  of the second embodiment, the width narrowing surface  107  has a step surface and a connection surface. 
     As a modification C7, a portion with the lowest height dimension Hc or the lowest cross-sectional area in the passage flow channel  61  may not be the outflow port  63   b . For example, the portion may be an intermediate portion between the flow channel boundary portion  64  and the outflow port  63   b  in the depth direction Z. Even in that case, if the height narrowing surface  105  is configured to narrow the passage flow channel  61 , a flow of the intake air in the measurement flow channel  62  can be appropriately accelerated. 
     As a modification C8, at least one of the inflow port  63   a  and the outflow port  63   b  may be entirely included in the parallel region  101 . For example, the outflow port  63   b  is configured to have only the first outlet area  63   b   1  of the first exit region  63   b   1  and the second exit region  63   b   2 . 
     As a modification C9, the downstream boundary portion  64   b  of the flow channel boundary portion  64  may be disposed on the bottom side of the upstream boundary portion  64   a  in the height direction Y. For example, the downstream boundary portion  64   b  is exposed to the upstream side from the inflow port  63   a  in the depth direction Z. Even in that configuration, if the outflow ceiling surface portion  66   b  extends in parallel with the depth direction Z, the configuration does not correspond to a configuration in which the outflow ceiling surface portion  66   b  narrows the passage flow channel  61  at a position exposed to the upstream side from the inflow port  63   a  in the depth direction Z. 
     As a modification C10, the passage bottom surface  67  may not have the parallel bottom surface portion  106 . For example, it is assumed that substantially the whole of the passage bottom surface  67  is the height narrowing surface  105 . In that configuration, the height narrowing surface  105  extends from the base end portion of the inflow restriction portion  85  toward the downstream side. In that case, the height narrowing surface  105  is in a state of being extended over the inflow restriction portion  85  and the outflow port  63   b.    
     Fourth Embodiment 
     In an air flow meter  50  according to a fourth embodiment, a flow channel boundary portion  64  is not exposed to an upstream side through an inflow port  63   a . In the present embodiment, similarly to the third embodiment, differences from the second embodiment will be mainly described. 
     As shown in  FIG. 33 , a flow channel forming portion  54  has a flow channel partition portion  111  that divides the passage path  61  to have a measurement flow channel  62  separate from a passage flow channel  61 . The flow channel partition portion  111  is provided on the downstream side of the flow channel boundary portion  64  in the depth direction Z and on the opposite side of a passage bottom surface  67  across the passage flow channel  61  in the height direction Y. A partition top portion  111   a , which is an upstream end portion of the flow channel partition portion  111 , serves as a downstream boundary portion  64   b  of the flow channel boundary portion  64 . In that case, the partition top portion  111   a  is located at the same position as that of the downstream boundary portion  64   b . A height dimension of the flow channel partition portion  111  gradually decreases as the flow channel partition portion  111  comes closer to the flow channel boundary portion  64  in the depth direction Z, and the smallest portion of the height dimension is the partition top portion  111   a . In that case, the partition top portion  111   a  is a top side extending in the width direction X. It can be conceivable that the flow channel partition portion  111  partitions the passage flow channel  61  and the measurement flow channel  62  vertically in the height direction Y. 
     The flow channel partition portion  111  is included in a housing main body  51   a  of a housing  51 . In the flow channel partition portion  111 , a surface facing the bottom side forms an outflow ceiling surface portion  66   b , and the surface facing the opposite side to the passage bottom surface  67  forms an inner peripheral surface of the measurement flow channel  62 . 
     The flow channel forming portion  54  has a ceiling projection portion  112  protruding toward the bottom side in addition to the flow channel partition portion  111 . The ceiling projection portion  112  is provided on the upstream side of the outflow ceiling surface portion  66   b . In the height direction Y, the ceiling top portion  112   a , which is the bottom-side end portion of the ceiling projection portion  112 , forms the upstream boundary portion  64   a  of the flow channel boundary portion  64 . In that case, it can be conceivable that the ceiling top portion  112   a  is located at the same position as that of the upstream boundary portion  64   a . A depth dimension of the ceiling projection portion  112  in the depth direction Z gradually decreases as the ceiling projection portion  112  comes closer to the passage bottom surface  67  in the height direction Y, and a portion having the smallest depth dimension becomes the ceiling top portion  112   a . In that case, the ceiling top portion  112   a  is a top side extending in the width direction X. 
     The ceiling projection portion  112  is included in the housing main body  51   a  of the housing  51 . In the ceiling projection portion  112 , a surface facing the upstream side in the depth direction Z forms the inflow ceiling surface portion  66   a , and the surface facing the downstream side forms the inner peripheral surface of the measurement flow channel  62 . 
     In the present embodiment, the partition top portion  111   a  is not exposed to the upstream side through the inflow port  63   a . For example, when a person looks into the passage flow channel  61  from the inflow port  63   a , even if the direction of the looking-in is changed, the partition top portion  111   a  cannot be visually recognized. In other words, the partition top portion  111   a  is hidden from the upstream side by the inflow restriction portion  85  and the ceiling projection portion  112 , and the sight line of the person from the inflow port  63   a  is blocked by the inflow restriction portion  85  and the ceiling projection portion  112 . The fact that the partition top portion  111   a  is not exposed means that the flow channel boundary portion  64  is also not exposed to the upstream side from the inflow port  63   a.    
     The inflow restriction portion  85  corresponds to a bottom projection portion protruding from the passage bottom surface  67  toward the ceiling side. The upper surface  85   b  of the inflow restriction portion  85  may be referred to as an upper end portion of the inflow restriction portion  85 , and if the upstream end portion of the upper surface  85   b  is referred to as a restriction top portion  85   c , the restriction top portion  85   c  is also included in the upper end portion of the inflow restriction portion  85 . 
     In the passage flow channel  61 , a virtual line connecting the restriction top portion  85   c  of the inflow restriction portion  85  and the ceiling top portion  112   a  of the ceiling projection portion  112  is referred to as a connecting line PL. The connecting line PL may also be referred to as a virtual line expressing the sight line that allows a person to see a portion close to the partition top portion  111   a  when looking into the passage flow channel  61  from the inflow port  63   a , for example. In addition, for example, in a configuration in which the multiple ceiling projection portions and the multiple bottom projection portions are present on the upstream side of the flow channel boundary portion  64 , a virtual line in which a connecting angle θa, which is an inclination angle with respect to the depth direction Z, has a maximum value, among the virtual lines connecting the tip portion of each ceiling projection portion and the tip portion of each bottom projection portion, is referred to as the connecting line PL. 
     When a virtual line extending in parallel with the depth direction Z is referred to as a depth reference line Za, the connecting angle θa is an angle of a portion facing toward the downstream side between the connecting line PL and the depth reference line Za. In that case, the connecting angle θa is a side in which the side where the downstream side portion of the connecting line PL is away from the passage bottom surface  67  increases with a positive value, and a side in which the side where the downstream side portion of the connecting line PL comes closer to the passage bottom surface  67  increases with a negative value. For that reason, as shown in  FIG. 33 , when the connecting line PL is inclined toward the downstream side so as to be away from the passage bottom surface  67 , the connecting angle θa has the positive value. On the other hand, when the connecting line PL is inclined toward the downstream side so as to comes closer to the passage bottom surface  67 , the connecting angle θa has the negative value. 
     The inner peripheral surface  61   a  of the passage flow channel  61  has an inflow upper end portion  113  and an outflow upper end portion  114 . The inflow upper end portion  113  is an end portion of the inflow port  63   a  opposite to the passage bottom surface  67  in the height direction Y, and the outflow upper end portion  114  is an end portion of the outflow port  63   b  opposite to the passage bottom surface  67  in the height direction Y. The inflow upper end portion  113  is located farther from the passage bottom surface  67  than the ceiling top portion  112   a  in the height direction Y. The inflow upper end portion  113  is located farther from the passage bottom surface  67  than the partition top portion  111   a  in the height direction Y. In this manner, since the inflow upper end portion  113  is disposed at a position spaced apart from the passage bottom surface  67  as far as possible, the open area of the inflow port  63   a  is set to be as large as possible. For that reason, it is inhibited that the amount of intake air flowing in from the inflow port  63   a  is insufficient and the detection accuracy of the flow rate detection unit  52  is lowered. 
     The outflow upper end portion  114  is located at a position closer to the bottom than the ceiling top portion  112   a  in the height direction Y. In this manner, since the outflow upper end portion  114  is disposed as close as possible to the passage bottom surface  67 , an open area of the outflow port  63   b  is set to be as small as possible. For that reason, since a pressure of the intake air flowing out from the outflow port  63   b  is increased, the intake air is likely to flow into the measurement flow channel  62 , and the amount of the intake air flowing into the measurement flow channel  62  is insufficient to inhibit the detection accuracy of the flow rate detection unit  52  from being lowered. In addition, the outflow upper end portion  114  is located farther from the passage bottom surface  67  than the restriction top portion  85   c  in the height direction Y. 
     In the height direction Y, the partition top portion  111   a  is disposed on the side opposite to the passage bottom surface  67  across the connecting line PL, so that the partition top portion  111   a  is not exposed to the upstream side from the inflow port  63   a . In that case, the connecting line PL passes between the partition top portion  111   a  and the passage bottom surface  67 , and the ceiling projection portion  112  enters between the partition top portion  111   a  and the restriction top portion  85   c . For that reason, as indicated by a solid line in  FIG. 34 , when a large foreign matter F5 that has entered the passage flow channel  61  from the inflow port  63   a  travels linearly along the connecting line PL, the large foreign matter F5 passes through the bottom side of the partition top portion  111   a  in the height direction Y, and is likely to collide with the outflow ceiling surface portion  66   b . As a result of that collision, the traveling direction of the large foreign matter F5 changes with respect to the height direction Y, but is likely to exit from the outflow port  63   b . In other words, the large foreign matter F5 traveling linearly in the passage flow channel  61  does not collide with the inner peripheral surface  61   a  of the passage flow channel  61 , and is less likely to enter the measurement flow channel  62  as it is. 
     On the other hand, unlike the present embodiment, in the configuration in which the partition top portion  111   a  is exposed to the upstream side from the inflow port  63   a  as shown in  FIG. 35 , when a large foreign matter F6 travels linearly along the connecting line PL, there is a concern that the large foreign matter F6 enters the measurement flow channel  62  as it is. In that case, the large foreign matter F6 does not collide with the inner peripheral surface  61   a  and enters the measurement flow channel  62  even though the traveling direction is not changed. In the above configuration, in the height direction Y, the connecting line PL passes through the side opposite to the passage bottom surface  67  across the partition top portion  111   a , and the upstream end portion of the measurement flow channel  62  and the flow channel boundary portion  64  are exposed to the upstream side from the inflow port  63   a . In that case, for example, when a person looks into the passage flow channel  61  from the inflow port  63   a , an inner peripheral surface of the measurement flow channel  62  or the flow channel boundary portion  64  can be visually recognized. 
     Returning to the description of  FIG. 33 , the passage flow channel  61  has a straight region  115 . The straight region  115  is a region linearly extending so as to connect the inflow port  63   a  and the outflow port  63   b , and the upstream end portion of the straight region  115  is included in the inflow port  63   a  and the downstream end is included in the outflow port  63   b . Unlike the parallel region  101  of the third embodiment, the straight region  115  is not parallel to the depth direction Z but inclined with respect to the depth direction Z. In the present embodiment, the straight region  115  is inclined with respect to the depth direction Z so as to come closer to the passage bottom surface  67  toward the downstream side. The inclination direction is opposite to the connecting line PL, and a straight angle θb indicating the inclination angle with respect to the depth direction Z has a negative value. The straight angle θb is an angle of a portion open to the downstream side between the straight region  115  and the reference line Za. On the other hand, similarly to the parallel region  101  of the third embodiment, a height dimension of the straight region  115  is uniform in any part in the depth direction Z. 
     As shown in  FIG. 34 , when a large foreign matter F7 flowing in from the inflow port  63   a  travels straight along the straight region  115 , the large foreign matter F7 exits from the outflow port  63   b  simply by traveling linearly along the straight region  115 . In this example, as described above, the inclination direction with respect to the depth direction Z is opposite between the straight region  115  and the connecting line PL. In that case, if the air flow meter  50  is installed in the intake passage  12  so that the amount of large foreign matter F7 traveling linearly along the straight region  115  increases among the foreign matter contained in the intake air, the number of foreign matter such as the large foreign matter F5 traveling along the connecting line PL itself is likely to decrease. This makes it easy to exert a deterrent force against foreign matter entering the passage flow channel  61  from the inflow port  63   a  entering the measurement flow channel  62  without colliding with the inner peripheral surface  61   a.    
     According to the present embodiment described so far, since the partition top portion  111   a  is not exposed to the upstream side through the inflow port  63   a , the foreign matter such as the large foreign matter F5 that travels linearly through the passage flow channel  61  does not collide with the inner peripheral surface  61   a  and enters the measurement flow channel  62  as it is, which is unlikely to occur. This makes it possible to inhibit the foreign matter from adhering to the flow rate detection unit  52  of the measurement flow channel  62 , and the detection accuracy of the flow rate detection unit  52  from being lowered by the foreign matter. 
     According to the present embodiment, since the partition top portion  111   a  and the downstream boundary portion  64   b  coincide with each other, a configuration in which the partition top portion  111   a  is not exposed to the upstream side through the inflow port  63   a  is realized, thereby being capable of realizing a configuration in which the flow channel boundary portion  64  is also not exposed to the upstream side through the inflow port  63   a . For that reason, the foreign matter such as the large foreign matter F5 traveling linearly through the passage flow channel  61  can be surely inhibited from directly entering the measurement flow channel  62  without colliding with the inner peripheral surface  61   a.    
     According to the present embodiment, since the connecting line PL passes through passage bottom surface  67  side of the partition top portion  111   a , a configuration in which the partition top portion  111   a  is not exposed to the upstream side through the inflow port  63   a  can be realized. 
     According to the present embodiment, the partition top portion  111   a  and the upstream boundary portion  64   a  coincide with each other. In other words, the upstream boundary portion  64   a  is not disposed on the bottom side of the partition top portion  111   a . For that reason, even though the partition top portion  111   a  is not exposed to the upstream side through the inflow port  63   a , the upstream end portion of the measurement flow channel  62  and the flow channel boundary portion  64  can be prevented from being exposed to the upstream side from the inflow port  63   a . As a result, the foreign matter such as the large foreign matter F5 that travels linearly along the connecting line PL can be inhibited from entering the measurement flow channel  62  as it is. 
     According to the present embodiment, since the ceiling top portion  112   a  is disposed at a height position between the partition top portion  111   a  and the restriction top portion  85   c  in the height direction Y, the straight region  115  can be secured by the passage flow channel  61 . In this example, unlike the present embodiment, for example, in a configuration in which the ceiling top portion  112   a  is disposed at a position closer to the passage bottom surface  67  than both of the partition top portion  111   a  and the restriction top portion  85   c  in the height direction Y, it is difficult to secure the straight region  115  in the passage flow channel  61  in an appropriate state. In addition, even in a configuration in which the ceiling top portion  112   a  is disposed at a position farther from the passage bottom surface  67  than both of the partition top portion  111   a  and the restriction top portion  85   c  in the height direction Y, it is similarly difficult to secure the straight region  115  in an appropriate state in the passage flow channel  61 . 
     On the other hand, according to the present embodiment, a positional relationship of the partition top portion  111   a , the restriction top portion  85   c , and the ceiling top portion  112   a  is set so that the straight region  115  can be secured in an appropriate state. For that reason, the foreign matter such as the large foreign matter F6 traveling linearly through the passage flow channel  61  can be inhibited from entering the measurement flow channel  62  as it is, and a configuration in which the foreign matter such as the large foreign matter F7 is urged to exit from the outflow port  63   b  as it is can be realized. Examples of the configuration that can secure the straight region  115  in the appropriate state include a configuration in which the inclination angle of the straight region  115  with respect to the depth direction Z does not become too large, a configuration in which the cross-sectional area of the straight region  115  does not become too small, and the like. 
     According to the present embodiment, since the inflow restriction portion  85  has a function of defining the angle of the connecting line PL as the bottom projection portion, there is no need to newly install a dedicated member or a dedicated portion for defining the angle of the connecting line PL in the passage flow channel  61 . This makes it possible to inhibit the complexity of the configuration of the air flow meter  50  and the tendency of disturbance to occur in the flow of the inflow air in the passage flow channel  61  due to the increase in the number of dedicated members and dedicated parts. 
     According to the present embodiment, since the partition top portion  111   a  is disposed at a position hidden on the depth side of the inflow restriction portion  85  and the ceiling projection portion  112  in the depth direction Z, the partition top portion  111   a  can be surely inhibited from being exposed on the upstream side from the inflow port  63   a . In that case, a configuration can be realized in which the partition top portion  111   a  is not exposed to the upstream side from the inflow port  63   a  by using the shapes of the passage ceiling surface  66  and the inflow port  63   a . For that reason, for example, there is no need to newly install a dedicated member or a dedicated portion for covering the partition top portion  111   a.    
     The fourth embodiment can be applied to various embodiments and combinations without departing from the scope of the present disclosure. 
     As a modification D1, the bottom projection portion does not have to be the inflow restriction portion  85 . For example, as shown in  FIG. 36 , the bottom projection portion  117  is provided at a position spaced downstream from the inflow port  63   a . The bottom projection portion  117  is provided on the upstream side of the ceiling top portion  112   a , and is disposed between the inflow port  63   a  and the ceiling top portion  112   a  in the depth direction Z. The bottom projection portion  117  has a bottom top portion  117   a  which is a tip portion of the bottom projection portion  117 , and even in that configuration, the connecting line PL connecting the ceiling top portion  112   a  and the bottom top portion  117   a  passes through the bottom side from the partition top portion  111   a . As a result, the partition top portion  111   a  is not exposed to the upstream side from the inflow port  63   a.    
     As a modification D2, the bottom top such as the restriction top portion  85   c  may be provided on the downstream side of the ceiling top portion  112   a . For example, in the modification D1, the bottom top portion  117   a  is provided on the downstream side of the ceiling top portion  112   a . In the above configuration, the bottom top portion  117   a  is disposed between the ceiling top portion  112   a  and the inflow port  63   a  in the depth direction Z, and the bottom projection portion  117  enters between the ceiling top portion  112   a  and the partition top portion  111   a . For example, the ceiling projection portion  112  is provided in the inflow port  63   a . Also, in the above configuration, the connecting line PL connecting the ceiling top portion  112   a  and the bottom top portion  117   a  passes through the bottom side of the partition top portion  111   a . On the other hand, the ceiling top portion  112   a  does not form the upstream boundary portion  64   a.    
     As a modification D3, the connecting line PL may be declined toward the bottom side toward the downstream side. In other words, the connecting angle θa may be a negative value. For example, as shown in  FIG. 37 , the restriction top portion  85   c  is disposed to be spaced apart from the passage bottom surface  67  more than the ceiling top portion  112   a . In the above configuration, the downstream end portion of the upper surface  85   b  of the inflow restriction portion  85  becomes the restriction top portion  85   c . Further, the inclination direction of the connecting line PL with respect to the depth direction Z is the same as the inclination direction of the straight region  115  with respect to the depth direction Z. Also in the above configuration, the partition top portion  111   a  is not exposed to the upstream side from the inflow port  63   a.    
     As a modification D4, the tip portion, which is the upstream end portion of the flow channel partition portion  111 , may have a flat tip end face. For example, as shown in  FIG. 38 , the tip end face  111   b  of the flow channel partition portion  111  is a flat surface, and the connecting line PL crosses the tip end face  111   b . An end portion of the tip end face  111   b  opposite to the bottom side is a partition top portion  111   a , and a bottom side end portion  111   c  opposite to the partition top portion  111   a  forms a downstream boundary portion  64   b . In this manner, although the partition top portion  111   a  and the downstream boundary portion  64   b  do not coincide with each other, even in this configuration, the partition top portion  111   a  is not exposed to the upstream side from the inflow port  63   a . In that case, for example, a large foreign matter F8 that travels linearly along the connecting line PL is likely to be returned from the measurement flow channel  62  to the passage flow channel  61  by colliding with the inner peripheral surface of the measurement flow channel  62  and rebounding even when the large foreign object F8 once enters the measurement flow channel  62  beyond the flow channel boundary portion  64 . In other words, the large foreign matter F8 is likely to exit from the outflow port  63   b.    
     As a modification D5, the straight region  115  may extend in parallel to the depth direction Z, similarly to the parallel region  101  of the third embodiment. Also in the above configuration, since the connecting line PL and the straight region  115  are relatively inclined, that is, the connecting angle θa and the straight angle θb are different from each other, a configuration can be realized in which the partition top portion  111   a  is not exposed to the upstream side from the inflow port  63   a.    
     As a modification D6, a part of the tip portion, which is the upstream end portion of the flow channel partition portion  111 , may be exposed to the upstream side from the inflow port  63   a . In this example, it is assumed that the tip end face of the flow channel partition portion  111  is flat or curved, so that a range of the tip portion cannot be clearly specified in the flow channel partition portion  111 , and the flow channel boundary portion  64  cannot also be clearly specified. 
     For example, as shown in  FIG. 39 , the tip end face  111   b  of the flow channel partition portion  111  and the connecting line PL intersect with each other. In the above configuration, an intersection angle θc between the connecting line PL and the tip end face  111   b  is greater than 90 degrees. The tip end face  111   b  is a curved surface that is curved so as to protrude toward the upstream side. In this example, a point at which the connecting line PL and the tip end face  111   b  intersect with each other is referred to as an intersection Ca, and a tangent line of the tip end face  111   b  at the intersection Ca is referred to as a partition tangent line TL, and the intersection angle θc is an angle of a portion that is open toward the downstream side between the connecting line PL and the partition tangent line TL. 
     In the above configuration, for example, as shown in  FIG. 40 , the large foreign matter F9 traveling linearly along the connecting line PL is likely to rebound to the bottom side toward the upstream side in the height direction Y after colliding with the tip end face  111   b  of the flow channel partition portion  111 . In other words, a large foreign matter F9 is likely to rebound toward the side opposite to the measurement flow channel  62 . This makes it possible to inhibit that the foreign matter such as the large foreign matter F9 is likely to rebound on the tip end face  111   b  and enter the measurement flow channel  62 . On the other hand, unlike the present modification D6, in the configuration in which the intersection angle θc is less than 90 degrees, it is considered that the large foreign matter F9 is likely to rebound toward the upstream side to the side opposite to the bottom side. In other words, it is considered that the large foreign matter F9 enters the measurement flow channel  62  by rebounding at the tip end face  111   b  of the flow channel partition portion  111 . 
     As a modification example D7, as shown in  FIG. 41 , in the configuration in which the tip end face  111   b  of the flow channel partition portion  111  is a curved surface, the connecting line PL passes through the bottom side from the partition center line Cb of the curvature. In the above configuration, the tip end face  111   b  and the partition center line Cb extend in parallel to the width direction X, and the flow channel boundary portion  64  is disposed at a position overlapping with the virtual line connecting the partition center line Cb and the ceiling top portion  112   a . In the above configuration, an angle between the tangent line of the tip end face  111   b  and the connecting line PL is greater than 90 degrees at a point where the connecting line PL and the tip end face  111   b  intersect with each other, as in the case of the modification D6. For that reason, the foreign matter that travels linearly along the connecting line PL is likely to travel to the opposite side of the measurement flow channel  62  by rebounding at the tip end face  111   b  of the flow channel partition portion  111 . For that reason, the foreign matter can be inhibited from entering the measurement flow channel  62 . 
     Fifth Embodiment 
     An air flow meter  50  according to a fifth embodiment has an guiding surface on which a foreign matter that is likely to travel linearly is brought toward one wall surface of a pair of wall surfaces in the width direction X. In the present embodiment, similarly to the third and fourth embodiments, differences from the second embodiment will be mainly described. 
     As shown in  FIGS. 42 to 44 , in the present embodiment, the pair of passage wall surfaces  68  in the second embodiment is a pair of passage wall surfaces  68   c  and  68   d , and those passage wall surfaces  68   c  and  68   d  correspond to passage facing surfaces. One front passage wall surface  68   c  is formed by a front cover  51   b  and a housing main body  51   a , and the other back passage wall surface  68   d  is formed by a back cover  51   c  and the housing main body  51   a . An inner peripheral surface  61   a  of the passage flow channel  61  has a guiding surface  121 . The guiding surface  121  is included in the inflow ceiling surface portion  66   a , and is provided in a state of being extended over the pair of passage wall surfaces  68   c  and  68   d , similarly to an inflow restriction portion  85 . In the width direction X, one end portion of the guiding surface  121  is disposed closer to the bottom than the other end. The housing main body  51   a  corresponds to a partition wall portion that partitions the passage flow channel  61  and the measurement flow channel  62  in the width direction X. 
     In the present embodiment, the guiding surface  121  has an end close to the front passage wall surface  68   c  is disposed to be closer to the bottom side than another end close to the back passage wall surface  68   d . In that case, the guiding surface  121  is an inclined surface gradually away from the bottom surface  67  as the guiding surface  121  comes closer to the back passage wall surface  68   d  in the width direction X. The inclination angle of the guiding surface  121  with respect to the width direction X is set to, for example, several degrees to several tens of degrees less than 45 degrees. A width dimension of the guiding surface  121  in the width direction X is larger than the height dimension in the height direction Y. The guiding surface  121  extends from an inflow port  63   a  toward the downstream side, and forms substantially the entire inflow ceiling surface portion  66   a.    
     The center line of the passage flow channel  61  is referred to as a passage center line CLa. The passage center line CLa is a virtual line connecting the center C1 of the inflow port  63   a  and the center C2 of the outflow port  63   b  (refer to  FIG. 42 ). The center line of the measurement flow channel  62  is referred to as a measurement center line CLb. The measurement center line CLb is a virtual line connecting the center C3 of the flow channel boundary portion  64  and a center C4 of a measurement outlet  63   c  (refer to  FIG. 44 ). In this example, a virtual line connecting the center C1 of the inflow port  63   a  and the center C4 of the measurement outlet  63   c  is referred to as a flow channel center line CL, and the flow channel center line CL includes the entire passage center line CLa and a part of the measurement center line CLb. The flow channel center line CL includes a connecting center line CLc as a virtual line connecting the passage center line CLa and the measurement center line CLb. The connection center line CLc is connected to the passage center line CLa by extending from the center C3 of the flow channel boundary portion  64  toward the upstream side of the passage flow channel  61 . 
     The inner peripheral surface  62   a  of the measurement flow channel  62  has a measurement ceiling surface  126 , a measurement bottom surface  127 , and a pair of measurement wall surfaces  128   a  and  128   b . The pair of measurement wall surfaces  128   a  and  128   b  are opposed to each other across the flow channel boundary portion  64  and the measurement outlet  63   c  in the width direction X, and correspond to branch facing surfaces. The front measurement wall surface  128   a  is formed by the front cover  51   b  and the housing main body  51   a , similarly to the front passage wall surface  68   c , and the back measurement wall surface  128   b  is formed by the back cover  51   c  and the housing main body  51   a , similarly to the back passage wall surface  68   d . The front measurement wall surface  128   a  has a width increasing surface  94 , and the back measurement wall surface  128   b  has a width decreasing surface  95 . The width increasing surface  94  and the width decreasing surface  95  are formed by the housing main body  51   a.    
     The measurement ceiling surface  126  extends from the downstream end portion of the inflow ceiling surface portion  66   a  toward the downstream side of the measurement flow channel  62 , and is in a state of being extended over the inflow ceiling surface portion  66   a  and the measurement outlet  63   c . The measurement bottom surface  127  extends from the upstream end portion of the outflow ceiling surface portion  66   b  toward the downstream side of the measurement flow channel  62 , and is in a state of being extended to the outflow ceiling surface portion  66   b  and the measurement outlet  63   c . In that case, the measurement ceiling surface  126  and the measurement bottom surface  127  face each other across the measurement wall surfaces  128   a  and  128   b.    
     In the present embodiment, in addition to the width direction X, the height direction Y, and the depth direction Z, the lateral direction α, the longitudinal direction β, and the flow channel direction γ are used to describe the configurations of the passage flow channel  61  and the measurement flow channel  62 . The lateral direction α has only a component in the width direction X. In the lateral direction α, a pair of passage wall surfaces  68   c  and  68   d  are aligned with each other, and a pair of measurement wall surfaces  128   a  and  128   b  are aligned with each other. The flow channel direction γ is basically a direction in which the passage flow channel  61  and the measurement flow channel  62  extend, does not have a component in the width direction X, and has a component in the height direction Y and a component in the depth direction Z. The longitudinal direction β is orthogonal to both the lateral direction α and the flow channel direction γ, and has no component in the width direction X, but has a component in the height direction Y and a component in the depth direction Z, similarly to the flow channel direction γ. In the longitudinal direction β, the passage ceiling surface  66  and the passage bottom surface  67  face each other, and the measurement ceiling surface  126  and the measurement bottom surface  127  face each other. The longitudinal direction β and the flow channel direction γ are different from the lateral direction α, and change at positions of the flow channels  61  and  62  because the passage flow channel  61  and the measurement flow channel  62  are curved. 
       FIG. 44  shows a diagram in which the passage flow channel  61  and the measurement flow channel  62  are extended along the flow channel center line CL with respect to the longitudinal direction β in the region between the inflow port  63   a  and the measurement outlet  63   b , when the measurement bottom surface  127  is viewed from the measurement ceiling surface  126  side. In  FIG. 43 , because the flow channel direction γ of the inflow port  63   a  coincides with the depth direction Z, the width direction X coincides with the lateral direction α, the height direction Y coincides with the longitudinal direction β, and the depth direction Z coincides with the flow channel direction γ. 
     As shown in  FIG. 44 , the passage flow channel  61  and the measurement flow channel  62  include an inflow region  131  and a lateral region  132 , and those regions  131  and  132  extend along the flow channel direction γ. The inflow region  131  is a region in which the inflow port  63   a  is projected in the flow channel direction γ, and extends from the inflow port  63   a  toward the measurement outlet  63   c . In the present embodiment, the inflow region  131  extends to the downstream end portion of the intermediate measurement path  92 . 
     The lateral region  132  is disposed side by side in the lateral direction α in the inflow region  131 . The lateral region  132  is disposed on the side of the front measurement wall surface  128   a , and the inflow region  131  is disposed on the side of the back measurement wall surface  128   b . The lateral region  132  is disposed on the downstream side of the width increasing surface  94  in the flow channel direction γ, and does not extend from the inflow port  63   a . For that reason, the lateral region  132  does not include a region in which the inflow port  63   a  is projected in the flow channel direction γ. The lateral region  132  is a region increased in the measurement flow channel  62 , including a portion in which the width dimension of each of the width increasing portion  91   a  and the intermediate measurement path  92  in the lateral direction α is larger than the width dimension of the upstream-side portion of the width increasing portion  91   a  in the upstream measurement path  91 . In the lateral direction α, the width dimension of the inflow region  131  is larger than the width dimension of the lateral region  132 . The width dimension of the inflow region  131  may be the same as or smaller than the width dimension of the lateral region  132 . In this description, the width dimensions of the portions having the largest width dimensions in each of the inflow region  131  and the lateral region  132  are compared with each other. 
     The upstream measurement path  91  corresponds to an upstream branch path, the intermediate measurement path  92  corresponds to an intermediate branch path, and the downstream measurement path  93  corresponds to a downstream branch path. Like the parallel region  101  and the like, the inflow region  131  and the lateral region  132  are virtual regions, and the passage flow channel  61  and the measurement flow channel  62  are not actually divided into the inflow region  131  and the lateral region  132 . Further, in  FIG. 44 , the inflow region  131  is illustrated by lighter dot hatching, and the lateral region  132  is illustrated by darker dot hatching. 
     The flow rate detection unit  52  is disposed in the lateral region  132  in the intermediate measurement path  92 . The measurement board portion  81   a  is disposed at a position extending across the inflow region  131  and the lateral region  132  in the lateral direction α so that the substrate surface on which the flow rate detection unit  52  is mounted is included in the lateral region  132 . The flow rate detection unit  52  is disposed at a position not overlapping with the inflow port  63   a  in the flow channel direction γ. In other words, the flow rate detection unit  52  is hidden from the upstream side by a portion forming the width increasing surface  94  in the housing main body  51   a  or the width increasing surface  94  in the flow channel direction γ. The measurement board portion  81   a  may be disposed in a position in which the entire measurement substrate portion is included in the lateral region  132 . 
     In the flow channel direction γ, a separation distance between the flow rate detection unit  52  and the width increasing surface  94  and a separation distance between the measurement board portion  81   a  and the width increasing surface  94  are both smaller than a length dimension of the width increasing surface  94 . As a result, the measurement board portion  81   a  and the flow rate detection unit  52  are disposed at positions relatively close to the width increasing surface  94 . An inclination angle of the width increasing surface  94  with respect to the flow channel direction γ is smaller than 45 degrees, for example. In that case, since a width dimension of the measurement flow channel  62  in the lateral direction α does not increase abruptly as the measurement flow channel  62  approaches the intermediate measurement path  92  but gradually increases, turbulence of the air flow such as a vortex is less likely to occur in the intake air reaching the lateral region  132 . 
     The guiding surface  121  of the passage flow channel  61  is inclined toward the back cover  51   c  in a state facing the bottom side, and thus is not orthogonal to the longitudinal direction β. The guiding surface  121  is gradually inclined toward the bottom as the inflow ceiling surface portion  66   a  comes closer to the flow channel boundary portion  64  as described above, so that the guiding surface  121  is exposed upstream from the inflow port  63   a  in the depth direction Z. Therefore, as shown in  FIG. 43 , when a large foreign matter F10 traveling linearly in the depth direction Z collides with the guiding surface  121 , the traveling direction of the large foreign matter F10 is changed toward the back passage wall surface  68   d  side and the passage bottom surface  67  side with respect to the width direction X and the height direction Y. In other words, the traveling direction of the large foreign matter F10 is not a direction parallel to the flow channel direction γ, but a direction inclined with respect to the flow channel direction γ so as to include the components of the lateral direction α and the longitudinal direction β. 
     Next, the way of how the foreign matter whose traveling direction is changed by the guiding surface  121  travels will be described with reference to  FIG. 45 . It should be noted the foreign matter entering the measurement flow channel  62  from the passage flow channel  61  is an object to be described hereafter, and therefore a description of a change in the traveling direction of the foreign matter in the longitudinal direction β will be omitted. In this example, both situations where the traveling direction of the foreign matter is changed and unchanged in the longitudinal direction β are assumed, and in either case, the foreign matter may be traveling along the flow channel direction γ. 
     As shown in  FIG. 45 , when large foreign matter F11, F12 traveling linearly in the flow channel direction γ collides with the guiding surface  121 , like the large foreign matter F10 described in  FIG. 43 , both the large foreign matter F11, F12 travel in a direction angled with respect to the flow channel direction γ toward the back cover  51   c . In this example, the large foreign matter F11 collides with the guiding surface  121  at a position closer to the front cover  51   b  in the lateral direction α, and the large foreign matter F12 collides with the guiding surface  121  at a position closer to the back cover  51   c . An inclination angle of the guiding surface  121  with respect to the lateral direction α is relatively small, and this causes the change in the traveling direction of the large foreign matter F11 and F12 by the guiding surface  121  to be relatively small. For that reason, the traveling directions of the large foreign matter F11 and F12 are changed by the guiding surface  121 , and then proceed along a flow of the intake air, so that the large foreign matter F11 and F12 is likely to coincide with each other in the flow channel direction γ again. 
     Specifically, the large foreign matter F11 that has collided with the guiding surface  121  proceeds obliquely from the position closer to the front cover  51   b  toward the back cover  51   c , and then proceeds in the flow channel direction γ by gradually changing the traveling direction by the flow of the intake air at a position before reaching the back cover  51   c . In that case, even when the large foreign matter F11 reaches the intermediate measurement path  92  and is closest to the measurement circuit board portion  81   a , the large foreign matter F11 passes through a position closer to the back cover  51   c  that is relatively distant from the measurement board portion  81   a  or the lateral region  132  in the lateral direction α. For that reason, even if the traveling direction of the large foreign matter F11 slightly changes in the direction facing the front cover  51   b  side, it is difficult for the large foreign matter F1 to enter from the inflow region  131  into the lateral region  132 . 
     On the other hand, unlike the present embodiment, for example, in a configuration in which the guiding surface  121  is not provided, the large foreign matter F11 passes through a position relatively close to the measurement board portion  81   a  or the lateral region  132  in the lateral direction α as it is as shown by a dashed line in  FIG. 45 . For that reason, even if the traveling direction of the large foreign matter F11 is slightly changed to the direction of the front cover  51   b , the large foreign matter F11 is likely to enter the lateral region  132  from the inflow region  131 . In that case, there is a concern that the large foreign matter F11 passes between the flow rate detection unit  52  and the front cover  51   b  and adheres to the flow rate detection unit  52 . 
     In addition, the large foreign matter F12 that has collided with the guiding surface  121  at a position closer to the back cover  51   c  than the large foreign matter F11 travels obliquely toward the back cover  51   c  as indicated by a solid line in  FIG. 45 , collides with the back cover  51   c , and accordingly travels obliquely toward the front cover  51   b . Thereafter, the large foreign matter F12 travels along the flow of the intake air at a position slightly distant from the back cover  51   c , thereby traveling in the flow channel direction γ. Even in that case, even when the large foreign matter F12 reaches the intermediate measurement path  92  and is closest to the measurement board portion  81   a , as in the case of the large foreign matter F11, the large foreign matter F12 passes through the position closer to the back cover  51   c  which is relatively distant from the measurement board portion  81   a  or the lateral region  132  in the lateral direction α. 
     According to the present embodiment described so far, since the flow rate detection unit  52  is provided in the lateral region  132  which is a region not projected along the flow channel direction γ from the inflow port  63   a , the foreign matter traveling in the inflow region  131  can be inhibited from reaching the flow rate detection unit  52 . In addition, since the guiding surface  121  for bringing the foreign matter away from the lateral region  132  in the lateral direction α is provided in the passage flow channel  61 , the foreign matter reaching the intermediate measurement path  92  is less likely to pass through a position close to the lateral region  132 . As a result, the foreign matter can be more surely inhibited from reaching the flow rate detection unit  52 . 
     According to the present embodiment, since the guiding surface  121  is extended over the pair of wall surfaces  128   a  and  128   b , in the passage flow channel  61 , the foreign matter can be brought in closer to the guiding surface  121  in the entire range in the lateral direction α. For that reason, the probability that the foreign matter that has entered the measurement flow channel  62  passes through the position close to the lateral region  132  in the lateral direction α can be reduced. 
     According to the present embodiment, since the guiding surface  121  is disposed on the upstream side of the flow channel boundary portion  64  in the passage flow channel  61 , the separation distance between the guiding surface  121  and the lateral region  132  in the flow channel direction γ can be appropriately ensured. In that case, after the traveling direction of the foreign matter has changed due to the collision of the foreign matter with the guiding surface  121 , a distance and a time for the traveling direction of the foreign matter to coincide with the flow channel direction γ again by the flow of the intake air can be secured until the foreign matter reaches the intermediate measurement path  92 . This makes it difficult for the foreign matter to reach the intermediate measurement path  92  and enter the lateral region  132  while the traveling direction of the foreign matter is inclined with respect to the flow channel direction γ by the guiding surface  121 . 
     According to the present embodiment, the width increasing surface  94  included in the front measurement wall surface  128   a  is gradually away from the back measurement wall surface  128   b  as the width increasing surface  94  comes closer to the measurement outlet  63   c , thereby forming the lateral region  132 . In that case, for example, as compared with a configuration in which the width increasing surface  94  extends in parallel with the lateral direction α, the turbulence such as a vortex flow is less likely to occur in the intake air reaching the lateral region  132 . For that reason, the foreign matter can be inhibited from entering the lateral region  132  by being entrained in the disturbance of the intake air. 
     According to the present embodiment, the inflow region  131  and the lateral region  132  are reserved by leveraging a difference in the structure that the upstream measurement path  91  is located between the housing main body  51   a  and the back cover  51   c , while the intermediate measurement path  92  is placed between the front cover  51   b  and the back cover  51   c . In that case, since there is no need to newly install a dedicated member or a dedicated portion for forming the lateral region  132  in the housing  51 , the structure of the housing  51  is avoided from becoming complicated, the disturbance of the flow of the intake air by the dedicated member or the like in the measurement flow channel  62 , and the like can be avoided. 
     In the present embodiment, a separation distance between the flow rate detection unit  52  and the width increasing surface  94  in the flow channel direction γ is smaller than a length dimension of the width increasing surface  94 . In other words, the flow rate detection unit  52  is disposed at a position relatively close to the width increasing surface  94 . In that configuration, when the foreign matter such as the large foreign matter F11 and F12 or the like reaches the intermediate measurement path  92 , the foreign matter immediately passes through the opposite side of the flow rate detection unit  52  across the measurement board portion  81   a . For that reason, the foreign matter is less likely to enter the lateral region  132  on the upstream side of the flow rate detection unit  52 . 
     The fifth embodiment can be applied to various embodiments and combinations without departing from the scope of the present disclosure. 
     As a modification E1, the guiding surface  121  may be included in the passage bottom surface  67  and the passage wall surfaces  68   c  and  68   d , instead of being included in the passage ceiling surface  66 . For example, a configuration in which the guiding surface  121  is included in the passage bottom surface  67  in a state of being extended over a pair of wall surfaces  68   c ,  68   d  is applied, or a configuration in which the guiding surface  121  is included in the front passage wall surface  68   c  is applied. In the configuration in which the guiding surface  121  is included in the front passage wall surface  68   c , the front passage wall surface  68   c  protrudes toward the back passage wall surface  68   d , and the guiding surface  121  is formed by a surface of the protruding portion on the back passage wall surface  68   d  side. Also in that configuration, the traveling direction of the foreign matter colliding with the guiding surface  121  is temporarily inclined toward the back cover  51   c , so that the position of the foreign matter in the lateral direction α can be moved to a position closer to the back cover  51   c.    
     As a modification E2, the guiding surface  121  may be provided at a position downstream of the inflow port  63   a  in the passage flow channel  61 . For example, the guiding surface  121  is provided at an intermediate position between the inflow port  63   a  and the flow channel boundary portion  64 . In that configuration, a part of the inflow ceiling surface portion  66   a  protrudes toward the bottom side at an intermediate position between the inflow port  63   a  and the flow channel boundary portion  64 , and the guiding surface  121  is formed by the bottom side surface of the protruding portion. 
     As a modification E3, the guiding surface  121  may be included in the inner peripheral surface  62   a  of the measurement flow channel  62 . For example, as shown in  FIGS. 46 and 47 , the guiding surface  121  is included in the measurement bottom surface  127 . In that configuration, the guiding surface  121  extends over the pair of measurement wall surfaces  128   a  and  128   b  in the lateral direction α. The guiding surface  121  extends from the flow channel boundary portion  64  to the width increasing surface  94  in the flow channel direction γ, and is formed almost entirely on the measurement bottom surface  127 . In that configuration, as compared with the configuration in which the guiding surface  121  is included in the inflow ceiling surface portion  66   a  as in the fifth embodiment, the separation distance between the lateral region  132  and the guiding surface  121  in the flow channel direction γ is reduced. For that reason, it is assumed that the foreign matter whose traveling direction is inclined toward the back cover  51   c  side by the guiding surface  121  passes through the flow rate detection unit  52  at a timing earlier than when the traveling direction coincides with the flow channel direction γ. Even in that case, since the entry of the foreign matter into the lateral region  132  is unlikely to occur, the detection accuracy of the flow rate detection unit  52  can be inhibited from being lowered due to the adhesion of the foreign matter or the like. 
     Also, in the modification E3, the modifications E1 and E2 may be applied, and the guiding surface  121  may be included in the measurement bottom surface  127  and the measurement wall surfaces  128   a  and  128   b  in the measurement flow channel  62 . 
     As a modification E4, the guiding surface  121  may be disposed on the downstream side of the flow channel boundary portion  64  in the passage flow channel  61 . For example, as shown in  FIGS. 48 and 49 , the guiding surface  121  is included in the outflow ceiling surface portion  66   b . Also in the above configuration, as in the fifth embodiment, the guiding surface  121  is extended over the pair of passage wall surfaces  68   c  and  68   d . The guiding surface  121  extends from the flow channel boundary portion  64  to the outflow port  63   b  in the flow channel direction γ, and is formed almost entirely on the outflow ceiling surface portion  66   b.    
     In the above modification E4, a virtual line connecting the center C2 of the outflow port  63   b  and the center C4 of the measurement outlet  63   c  is referred to as an outflow center line CM. The outflow center line CM includes a return center line CLd as a virtual line connecting the passage center line CLa and the measurement center line CLb. The return center line CLd is connected to the passage center line CLa by extending from the center C3 of the flow channel boundary portion  64  toward the downstream side in the passage flow channel  61 . 
     In the above configuration, even if the foreign matter traveling through the passage flow channel  61  returns to the upstream side and enters the measurement flow channel  62  due to collision with the outflow ceiling surface portion  66   b , the position of the foreign matter is easily changed to a position closer to the back cover  51   c  so as to move away from the lateral region  132 . For that reason, the foreign matter that has returned from the outflow port  63   b  to the upstream side and has entered the measurement flow channel  62  is also less likely to pass through the position close to the lateral region  132  when reaching the intermediate measurement path  92 , as in the fifth embodiment. 
     In the above modification E4, the above-described modifications E1 and E2 may be applied, and the guiding surface  121  may be included in the passage bottom surface  67  and the passage wall surfaces  68   c  and  68   d  on the downstream side of the flow channel boundary portion  64  in the passage flow channel  61 . 
     As a modification E5, multiple guiding surfaces  121  may be provided. For example, as shown in  FIG. 50 , the guiding surface  121  is included in each of the inflow ceiling surface portion  66   a , the outflow ceiling surface portion  66   b , and the measurement bottom surface  127 . In the above configuration, the foreign matter that has entered the measurement flow channel  62  from the upstream side in the passage flow channel  61  can be brought in closer to the back cover  51   c  by the two guiding surfaces  121 . In addition, both of the foreign matter that has entered the measurement flow channel  62  by returning from the downstream side to the upstream side can be positioned toward the back cover  51   c  by the three guiding surfaces  121 . This makes it possible to more reliably inhibit the foreign matter that has reached the intermediate measurement path  92  from entering the lateral region  132 . 
     As a modification E6, a covering portion  136  may be provided to cover the flow rate detection unit  52  from the upstream side. For example, as shown in  FIGS. 51 and 52 , the cover portion  136  is provided in the measurement flow channel  62 . In the above configuration, the cover portion  136  is disposed at a position spaced downstream from the inflow port  63   a  in the flow channel direction γ, and the cover portion  136  is disposed between the inflow port  63   a  and the lateral region  132 . In that case, the lateral region  132  is hidden downstream of the covering portion  136  so that the inflow port  63   a  is not included in a region projected in the flow channel direction γ. In the passage flow channel  61  and the measurement flow channel  62 , a region formed closer to the inflow port  63   a  than the cover portion  136  is referred to as a near-side region  134 . The near-side region  134  is disposed laterally with the lateral region  132  in the lateral direction α in the inflow region  131 . 
     The cover portion  136  has a cover surface  136   a  and an orthogonal surface  136   b . The cover surface  136   a  has a function of guiding the foreign matter advancing toward the downstream side to the back cover  51   c , and faces the back cover  51   c  side. The cover surface  136   a  is an inclined surface that moves away from the back cover  51   c  as the cover surface  136   a  comes closer to the inflow port  63   a , and is inclined so as to face the inflow port  63   a  side with respect to the flow channel direction γ. In the lateral direction α, a width dimension of the cover portion  136  gradually decreases as the cover portion  136  approaches the inflow port  63   a . The cover portion  136  is included in the housing main body  51   a , and the cover surface  136   a  is included in the front measurement wall surface  128   a . The inclination angle of the cover surface  136   a  with respect to the front cover  51   b  is, for example, several degrees to several tens of degrees smaller than 45 degrees. 
     The orthogonal surface  136   b  is orthogonal to the flow channel direction γ and faces the measurement outlet  63   c  in the flow channel direction γ. In the flow channel direction γ, the flow rate detection unit  52  is disposed between the orthogonal surface  136   b  and the measurement outlet  63   c . The orthogonal surface  136   b  extends parallel to the lateral direction α, but may be inclined with respect to the lateral direction α. 
     In the above configuration as well, as in the fifth embodiment, when the large foreign matter F11 and F12 traveling linearly in the flow channel direction γ collides with the guiding surface  121 , as shown in  FIG. 52 , both the large foreign matter F11 and F12 travel in a direction inclined with respect to the flow channel direction γ toward the back cover  51   c . For that reason, even when the large foreign matter F11 and F12 reach the intermediate measurement path  92  and is closest to the measurement board portion  81   a , the large foreign matter F11 and F12 passes through a position in the lateral direction α, which is relatively distant from the lateral region  132 , from the measurement board portion  81   a . Even if the foreign matter such as the large foreign matter F11 advances in the near-side region  134  instead of the inflow region  131 , the foreign matter is guided to the back cover  51   c  side by colliding with the cover surface  136   a . In other words, the foreign matter is guided to a position away from the lateral region  132  in the lateral direction α. This makes it possible to inhibit the foreign matter advancing in the inflow region  131  and the foreign matter advancing in the near-side region  134  from entering the lateral region  132 . 
     As a modification E7, the guiding surface  121  may not bring the foreign matter closer to the inflow region  131  in the lateral direction α, but may bring the foreign matter closer to the lateral region  132 . In other words, the guiding surface  121  may face the front cover  51   b  instead of the back cover  51   c . In the above configuration, as shown in  FIG. 53 , when the large foreign matter F11 and F12 traveling linearly in the flow channel direction γ collides with the guiding surface  121 , the large foreign matter F11 and F12 travels in a direction inclined with respect to the flow channel direction γ toward the front cover  51   b , which is opposite to the fifth embodiment. In that case, the large foreign matter F11 and F12 reaches the cover surface  136   a  by traveling through the near-side region  134  instead of the inflow region  131 , and is likely to be guided to the back cover  51   c  side by colliding with the cover surface  136   a . For that reason, the large foreign matter F11 and F12 passes through a position relatively far from the lateral region  132 , and therefore, the large foreign matter F11 and F12 can be inhibited from entering the lateral region  132 . 
     As a modification F8, the flow rate detection unit  52  may not be separated from the width increasing surface  94  toward the measurement outlet  63   c  in the flow channel direction γ, but at least a part of the flow rate detection unit  52  may be aligned with the width increasing surface  94  in the lateral direction α. In that case, since the flow rate detection unit  52  can be disposed in the immediate vicinity of the width increasing surface  94 , the foreign matter can be more surely inhibited from entering the lateral region  132  at a position before passing through the flow rate detection unit  52  in the inflow region  131 . 
     Sixth Embodiment 
     An air flow meter  50  according to a sixth embodiment has inflow step surfaces  71   a  of the second embodiment, a parallel region  101  and a height narrowing surface  105  of the third embodiment, a configuration in which a partition top portion  111   a  of the fourth embodiment is not exposed from an inflow port  63   a , and a lateral region  132  of the fifth embodiment. In the present embodiment, differences from the fifth embodiment will be mainly described. 
     As shown in  FIGS. 54 and 55 , in the present embodiment, unlike the second embodiment, all the inflow step surfaces  71   a  are not orthogonal to the depth direction Z, but are inclined with respect to the depth direction Z. In that case, the inflow step surfaces  71   a  are inclined with respect to the width direction X, but are not inclined with respect to the height direction Y, and extend parallel to the height direction Y. The inflow step surfaces  71   a  are inclined so that an end portion on a front passage wall surface  68   c  side is disposed at a position closer to the inflow port  63   a  than an end portion on a back passage wall surface  68   d  side, and the inflow step surfaces  71   a  are inclined surfaces facing the inflow port  63   a  and the back passage wall surface  68   d . The inclination angle of the inflow step surface  71   a  with respect to the width direction X is set to, for example, several degrees to several tens of degrees smaller than 45 degrees. 
     The inflow step surfaces  71   a  have a function as an guiding surface  121  of the fifth embodiment. For example, when a large foreign matter that travels linearly in the depth direction Z collides with the inflow step surfaces  71   a  and rebounds, the large foreign matter does not travel parallel to the depth direction Z toward the inflow port  63   a , but travels toward the back passage wall surface  68   d . In that case, for example, even if the large foreign matter rebounded at the inflow step surfaces  71   a  advances in the passage flow channel  61  again toward the downstream side by the flow of the intake air, the large foreign matter advances to a position closer to the back passage wall surface  68   d . As described above, the inflow step surface  71   a  corresponds to an guiding surface, and as shown in  FIG. 56 , the provision of the multiple inflow step surfaces  71   a  in an inflow ceiling surface portion  66   a  corresponds to the provision of the multiple guiding surfaces. 
     Like the large foreign matter F11 and F12 of the fifth embodiment, the large foreign matter traveling in the flow channel direction γ at the position close to the back passage wall surface  68   d  passes through a position relatively distant from the flow rate detection unit  52  in the lateral direction α even if the large foreign matter reaches an intermediate measurement path  92 . For that reason, even if the traveling direction of the large foreign matter slightly changes in the direction facing the front cover  51   b , the large foreign matter is unlikely to enter from the inflow region  131  into the lateral region  132 . 
     In the present embodiment, the inflow ceiling surface portion  66   a  partitioning the ceiling-side region  102  has an inflow step surface  71   a . In that case, the foreign matter that has entered the ceiling-side region  102  from the inflow port  63   a  is rebounded to the inflow port  63   a  side at the inflow ceiling surface portion  66   a , thereby inhibiting the passage of the foreign matter to the downstream side of the ceiling-side region  102  in the passage flow channel  61 . In addition, the foreign matter that has entered the parallel region  101  from the inflow port  63   a  advances linearly in the depth direction Z as it is, so that the foreign matter easily comes out from the outflow port  63   b  to the outside. Further, even with respect to the foreign matter that travels linearly in the direction inclined with respect to the depth direction Z, the partition top portion  111   a  is not exposed to the upstream side from the inflow port  63   a , which makes it difficult to directly enter the measurement flow channel  62  in the state of maintaining the linearly traveling. Even if there is a foreign matter that has entered the measurement flow channel  62 , the foreign matter is likely to come closer to a position closer to the back cover  51   c  by colliding with the inflow step surfaces  71   a  that function as the guiding surface. For that reason, the foreign matter reaching the intermediate measurement path  92  is inhibited from entering the lateral region  132 . 
     The sixth embodiment can be applied to various embodiments and combinations without departing from the scope of the present disclosure. 
     As a modification F1, the function as the guiding surface may be imparted to an inflow connection surface  72   a . For example, like the guiding surface  121  of the fifth embodiment, the inflow connection surface  72   a  may not be orthogonal to the height direction Y, but may be an inclined surface facing the passage bottom surface  67  side and the back cover  51   c  side. In addition, the function as the guiding surface may be imparted to the outflow step surfaces  71   b  or the outflow connection surface  72   b.    
     As a modification F2, not all of the inflow step surfaces  71   a  may be provided with a function as the guiding surface, but at least one of the inflow step surfaces  71   a  may be provided with a function as the guiding surface. For example, the inflow step surface  71   a  disposed at the most downstream side of the multiple inflow step surfaces  71   a  is inclined with respect to the depth direction Z, thereby serving as the guiding surface. In the above configuration, the other inflow step surfaces  71   a  are orthogonal to the depth direction Z and do not have the function as the guiding surface. 
     A modification F3 may have at least two configurations among the inflow step surfaces  71   a  of the second embodiment, the parallel region  101  and the height narrowing surface  105  of the third embodiment, the configuration in which the partition top portion  111   a  is not exposed from the inflow port  63   a  in the fourth embodiment, and the lateral region  132  of the fifth embodiment. Even in the above case, a deterrent force against the foreign matter reaching the flow rate detection unit  52  can be exerted. Although the multiple embodiments according to the present disclosure have been described above, the present disclosure is not construed as being limited to the above-mentioned embodiments, and can be applied to various embodiments and combinations within a range not departing from the spirit of the present disclosure. 
     As a modification example G1, in each of the embodiments described above, the flow rate detection unit is provided in the measurement flow channel as the physical quantity detector, but the physical quantity detector provided in the measurement flow channel may be a humidity detection unit, a temperature detection unit, or a pressure detection unit. 
     As a modification G2, in each of the above-mentioned embodiments, the measurement flow channel has a circulating shape, but the measurement flow channel may have a shape extending in the depth direction Z without circulating. 
     Although the present disclosure has been described in accordance with the examples, it is understood that the disclosure is not limited to such examples or structures. The present disclosure encompasses various modifications and variations within the scope of equivalents. In addition, various combinations and configurations, as well as other combinations and configurations that include only one element, more, or less, are within the extent and spirit of the present disclosure.