Patent Publication Number: US-7709769-B2

Title: Steam cooker

Description:
TECHNICAL FIELD 
     This invention relates to a steam cooker. 
     BACKGROUND ART 
     Heretofore, as a steam cooker for cooking an object to be cooked such as food by using steam, there is a one that feeds superheated steam into an oven (for example, see JP 8-49854 A). This steam cooker has a steam generator in which a heater is provided in a pot to generate steam, and a steam superheater that generates superheated steam by heating the steam generated by the steam generator, and the superheated steam generated by the steam superheater is sent into the oven to cook food. 
     Incidentally, in the conventional steam cooker, superheated steam generated by the steam superheater blows into the oven through a steam outlet provided in an upper portion of a side surface of the oven, the superheated steam is sucked through a suction opening provided in a lower portion of the side surface, so that the superheated steam is circulated in the oven and in a circulation air passage. Therefore, in the conventional steam cooker, airflow in the oven is biased, and temperature distribution does not become uniform, so that uneven cooking occurs in an object to be cooked, and cooking with a good finish cannot be done 
     SUMMARY OF THE INVENTION 
     An object of this invention is, therefore, to provide a steam cooker that can make temperature distribution in a heating chamber uniform and ensure even cooking. 
     In order to accomplish the above object, a steam cooker according to the present invention comprises: 
     a steam generator for generating steam; 
     a steam temperature-raising device for raising a temperature of steam coming from the steam generator; 
     a heating chamber provided with a ceiling steam outlet on its ceiling side, in which an object to be cooked is heated by steam supplied from the steam temperature-raising device via the ceiling steam outlet, 
     the steam temperature-raising device including:
         a tray-shaped case having a recessed part which has a sidewall provided with a steam supply port through which steam from the steam generator enters the recessed part, the recessed part having a planar shape that is roughly symmetrical with respect to a center line of a steam flow entering through the steam supply port, the tray-shaped case being placed, with an opening of the recessed part directed downward, on the heating chamber at the ceiling steam outlet;   a heater placed in the recessed part such that a planar shape of the heater is roughly symmetrical with respect to said center line; and   a spiral heat radiation fin wound around the heater and having a fin pitch of 10 mm or less.       

     In the steam cooker with the above construction, steam generated by the steam generator is raised in temperature by the steam temperature-raising device to become superheated steam, and the superheated steam is supplied into the heating chamber, whereby the object to be cooked in the heating chamber is heated. At this time, steam generated in the steam generator enters the recessed part of the tray-shaped case of the steam temperature-raising device through the steam supply port provided in the sidewall of the recessed part which is placed above the ceiling steam outlet of the heating chamber. Steam flowing into the recessed part, which has a planar shape (i.e., a shape in a plan view) roughly symmetrical with respect to the center line of the steam flow entering through the steam supply port, is divided into the left and right side branches after colliding with a sidewall on the opposite side of the steam supply port of the recessed part, and a part of the branched steam returns along the sidewalls of the recessed part toward the steam supply port and merges into steam entering through the steam supply port. In this manner, steam filling the recessed part of the tray-shaped case is supplied into the heating chamber via the ceiling steam outlet of the heating chamber located on the opening side of the recessed part. 
     The planar shape, i.e., the shape in a plan view, of the recessed part of the tray-shaped case and the planar shape of the heater placed in the recessed part are individually made roughly symmetrical with respect to the center line of the steam flow entering by the steam supply port, whereby the steam flow in the recessed part is not biased to the left or right, so that the temperature distribution in the entire recessed part of the tray-shaped case is made uniform. 
     Further, the fin pitch of the spiral heat radiation fin wound around the heater, which heater is provided in the recessed part of the tray-shaped case and arranged in a planar shape roughly symmetrical with respect to the center line, is set to 10 mm or less, whereby influence (disturbance or resistance loss) of the heat radiation fin on the steam flow jetted from the steam supply port can be distributed in a manner so as to be roughly symmetrical with respect to the center line, and the steam temperature distribution in the recessed part is made uniform, while improving the efficiency of heat radiation from the heater by the heat radiation fin. Therefore, by making the temperature distribution of steam supplied into the heating chamber through the ceiling steam outlet of the heating chamber uniform, the temperature distribution in the heating chamber can be made uniform, and uniform cooking can be realized. 
     In one embodiment, a plurality of steam outlets for supplying steam into the heating chamber sideways are provided, in roughly symmetrical arrangement with respect to said center line, in sidewalls facing each other across the center line of the recessed part of the tray-shaped case and on the opposite side of the steam supply port. 
     In the steam cooker of the above embodiment, the plurality of steam outlets are provided in the sidewalls facing each other across the center line of the recessed part of the tray-shaped case and on the opposite side of the steam supply port, and a part of steam in the recessed part is supplied sideways into the heating chamber via the plurality of the steam outlets. By roughly symmetrically providing the plurality of steam blowout openings in the sidewalls facing each other across the center line of the recessed part of the tray-shaped case and on the opposite side of the steam supply port, the temperature distribution of steam that blows outward toward both sides of the center line via the plurality of steam outlets can be made uniform. Thus, the bias to the left or right in the temperature distribution of steam supplied into the heating chamber sideways can be reduced, so that the temperature distribution in the heating chamber can be made uniform. Furthermore, by providing the plurality of steam outlets in the sidewalls facing each other across the center line of the recessed part of the tray-shaped case and on the opposite side of the steam supply ports, steam of the highest flow rate and temperature (due to heating by the heater) in the recessed part blows out from the plurality of steam outlets. This vigorously supplies high-temperature steam into the heating chamber. Thereby, steam in the heating chamber is agitated, which is preferred in resolving uneven cooking. 
     In one embodiment, a sidewall on the opposite side of the steam supply port of the recessed part of the tray-shaped case is inclined with respect to a plane roughly perpendicular to said center line. 
     In the steam cooker of the above embodiment, since the sidewalls on the opposite side of the steam supply port in the recessed part of the tray-shaped case are inclined with respect to the plane that is roughly perpendicular to the center line, when steam flowing into the recessed part collides with the sidewall on the opposite side of the steam supply port in the recessed part and is divided, steam branch flows are made smooth and generation of steam accumulation and so on causing uneven temperature distribution is suppressed. As a result, the temperature distribution of steam that flows in the recessed part can be made uniform. 
     In one embodiment, the planar shape of the recessed part of the tray-shaped case is a generally pentagonal shape having three sides forming a U-shape and two sides continuous with both ends of the U-shape of the three sides and forming a generally V-shape bent outward, and the steam supply port is provided in a sidewall corresponding to a central one of the three sides forming the U-shape of the recessed part of the tray-shaped case. 
     In the steam cooker of the above embodiment, in the recessed part having a roughly pentagonal planar shape, the two sides forming the V-shape projecting outward, which are continuous with both the ends of the U-shape formed by the three sides, serve as the sidewall that are inclined with respect to the plane roughly perpendicular to the center line. Thereby, when steam flows into the recessed part through the steam supply port provided in the sidewall corresponding to a central one of the three sides forming the U-shape of the recessed part of the tray-shaped case, and collides with the two sidewalls forming the V-shape on the opposite side of the steam supply port, and is bifurcated to the left and right, the steam branch flows are made smooth, and generation of steam accumulation and so on causing uneven temperature distribution is suppressed, so that the temperature distribution of steam flowing in the recessed part can be made uniform. 
     In one embodiment, the planar shape of the recessed part of the tray-shaped case is a shape having three sides forming a U-shape and an arc-shaped outer periphery that is continuous with both ends of the U-shape of the three sides and curves outward, and the steam supply port is provided in a sidewall corresponding to a central one of the three sides forming the U-shape of the recessed part of the tray-shaped case. 
     In the steam cooker of the above embodiment, in the recessed part of the tray-shaped case, the arc-shaped outer periphery continuous with both the ends of the U-shape formed by the three sides and curving outward, serves as the sidewall that is inclined with respect to the plane roughly perpendicular to the center line. Thereby, when steam flows into the recessed part through the steam supply port provided in the sidewall corresponding to a central one of the three sides forming the U-shape of the recessed part of the tray-shaped case, and collides with the sidewall forming the arc-shaped outer periphery on the opposite side of the steam supply port, and is bifurcated to the left and right, the steam branch flows are made smooth, and generation of steam accumulation and so on causing uneven temperature distribution is suppressed, so that the temperature distribution of steam flowing in the recessed part can be made uniform. 
     In one embodiment, each of corners of the recessed part of the tray-shaped case has a curved surface so that adjacent sidewalls are smoothly continuous with each other. 
     In the steam cooker of the above embodiment, the curved surface is provided at each corner of the recessed part of the tray-shaped case so that the adjacent sidewalls are smoothly continuous, whereby the part of steam flowing into the recessed part flows along the sidewalls of the recessed part smoothly. Therefore, the steam flow in the recessed part is stabilized, thus enabling the temperature distribution of steam flowing in the recessed part to be more uniform. 
     As is apparent from the above description, according to the steam cooker of the invention, a steam cooker that can make a temperature distribution in a heating chamber uniform and secure cooking with no uneven heating is realized. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing an external appearance of a steam cooker according to one embodiment of the invention; 
         FIG. 2  is a perspective view showing an external appearance of the steam cooker with its door opened; 
         FIG. 3  is a schematic diagram showing the construction of the steam cooker; 
         FIG. 4(   a ) is a plan view of a pot of a steam generator of the steam cooker, and  FIG. 4(   b ) is a side view of the pot; 
         FIG. 5(   a ) is a side view of the steam generator  40 , and  FIG. 5(   b ) is a cross sectional view taken along line V-V of  FIG. 5(   a ); 
         FIG. 6  is a control block diagram of the steam cooker; 
         FIG. 7(   a ) is a plan view, seen from below, of a steam temperature-raising device of the steam cooker, and  FIG. 7(   b ) is a side view of the steam temperature-raising device, seen from a side of steam supply ports; 
         FIG. 8(   a ) is a plan view, seen from below, of a steam temperature-raising device not having a second steam superheater, and  FIG. 8(   b ) is a side view of the steam temperature-raising device, seen from a side of steam supply ports; 
         FIG. 9(   a ) is a schematic view showing a steam flow in the steam temperature-raising device, and  FIG. 9(   b ) is a schematic view showing another example of a tray-shaped case of the steam temperature-raising device; 
         FIG. 10  is a schematic view showing a steam flow in another steam temperature-raising device; 
         FIG. 11(   a ) is a view for explaining heat radiation fins of the steam temperature-raising device of the steam cooker according to the embodiment of the invention, and  FIG. 11(   b ) is a view for explaining heat radiation fins having a wider pitch than that of the heat radiation fins of  FIG. 11(   a ); 
         FIG. 12  is a front elevational view of the steam cooker with its door opened; and 
         FIG. 13  is a view for explaining another example of a heater to be used in the steam temperature-raising device of the steam cooker. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The steam cooker of the invention will be described using examples shown in the drawings. 
       FIG. 1  is an external perspective view of a steam cooker  1  according to one embodiment of the invention. In the steam cooker, a front upper portion of a rectangular parallelepiped cabinet  10  is provided with an operation panel  11 , and a door  12 , which is rotatable around a lower end side of the cabinet, is provided under the operation panel  11 . An upper portion of the door  12  is provided with a handle  13 , and the door  12  is provided with a window  14  made of thermal glass. 
       FIG. 2  is an external perspective view of the steam cooker  1  with the door  12  opened. A rectangular parallelepiped heating chamber  20  is provided in the cabinet  10 . The heating chamber  20  has an opening  20   a  on its front side facing the door  12 , and side surfaces, a bottom surface and a top surface of the heating chamber  20  are formed of stainless steel plates. A side of the door  12  facing the heating chamber  20  is formed of a stainless steel plate. A heat insulator (not shown) is placed in the surrounding of the heating chamber  20  and the interior of the door  12  to insulate the inside of the heating chamber  20  from the outside. 
     A stainless steel-made catch pan  21  is placed at the bottom surface of the heating chamber  20 , and a stainless steel wire-made rack  24  (shown in  FIG. 3 ) for receiving an object to be cooked is placed on the catch pan  21 . Further, side steam outlets  22 ,  22  of a roughly rectangular shape, which are longitudinally roughly level to each other (only one of the openings is shown in  FIG. 2 ), are provided at both lateral side surfaces of the heating chamber  20 . 
       FIG. 3  is a schematic view showing the basic construction of the steam cooker  1 . As shown in  FIG. 3 , the steam cooker X includes the heating chamber  20 , a water tank  30  for storing water for steam, a steam generator  40  for evaporating water supplied from the water tank  30 , a steam temperature-raising device  50  for heating steam from the steam generator  40  and a control unit  80  for controlling the steam generator  40 , the steam temperature-raising device  50  and other devices. 
     The lattice-like rack  24  is placed on the catch pan  21  placed in the heating chamber  20 , and an object  90  to be cooked is placed at a roughly central portion of the rack  24 . 
     A connecting portion  30   a  provided at the lower side of the water tank  30  is connected to a funnel-like receiving port  31   a  provided at one end of a first water supply pipe  31 . The suction side of a pump  35  is connected to an end of a second water supply pipe  32 , which branches off from the first water supply pipe  31  and extends upward, and one end of a third water supply pipe  33  is connected to the discharge side of the pump  35 . A water level sensor  36  for the water tank is provided at an upper end portion of a pipe  38  for the water level sensor, which pipe branches off from the first water supply pipe  31  and extends upward. Further, an upper end portion of an air releasing pipe  37  that branches off from the first water supply pipe  31  and extends upward is connected to an exhaust gas duct  65 . 
     The third water supply pipe  33  has an L shape that is bent roughly horizontally from a vertically placed portion and an auxiliary tank  39  is connected to the other end of the third water supply pipe  33 . One end of a fourth water supply pipe  34  is connected to a lower end of the auxiliary tank  39 , and the other end of the fourth water supply pipe  34  is connected to a lower end of the steam generator  40 . One end of a drain valve  70  is connected to the lower end of the steam generator  40 , to which the fourth water supply pipe  34  is connected at one end thereof. One end of a drain pipe  71  is connected to the other end of the drain valve  70 , and a water drain tank  72  is connected to the other end of the drain pipe  71 . An upper portion of the auxiliary tank  39  communicates with air via the air releasing pipe  37  and the exhaust gas duct  65 . 
     Once the water tank  30  has been connected, water rises in the air releasing pipe  37  until its water level reaches the same water level as that of the water tank  30 . Since a tip of the pipe  38  connected to the water level sensor  36  for the water tank is sealed, the water level in the pipe  38  does not rise, but a pressure in a sealed space in the pipe  38  for the water level sensor increases from an atmospheric pressure, depending on the water level of the water tank  30 . This pressure change is detected by a pressure detection device (not shown) in the water level sensor  36  for the water tank, whereby the water level in the water tank  30  is detected. Although water level measurement does not require the air releasing pipe  37  while the pump  35  is stationary, the air releasing pipe  37  having an open end is used in order to prevent deterioration of accuracy in the detection of the water level due to direct application of a suction pressure of the pump  35  to the pressure detection device. 
     The steam generator  40  has a pot  41 , to the lower side of which the other end of the fourth water supply pipe  34  is connected, a heater  42  placed in the vicinity of a bottom surface in the pot  41 , a water level sensor  43  placed in the vicinity of the upper side of the heater  42  in the pot  41 , and a steam suction ejector  44  attached to the upper side of the pot  41 . A fan casing  26  is placed outside an intake opening  25  provided at an upper portion of a lateral side of the heating chamber  20 . Steam in the heating chamber  20  is sucked through the intake opening  25  by a blower fan  28  placed in the fan casing  26 . The sucked steam is sent to an inlet side of the steam suction ejector  44  via a first pipe  61  and a second pipe  62 . The first pipe  61  is placed roughly horizontally, and its one end is connected to the fan casing  26 . The second pipe  62  is roughly vertically placed, and its one end is connected to the other end of the first pipe  61 , and the other end of the second pipe  62  is connected to the inlet side of an inner nozzle  45  of the steam suction ejector  44 . 
     The steam suction ejector  44  is provided with an outer nozzle  46  that covers an outer side of the inner nozzle  45 , and the discharge side of the inner nozzle  45  communicates with an internal space of the pot  41 . The discharge side of the outer nozzle  46  of the steam suction ejector  44  is connected to one end of a third pipe  63  and a steam temperature-raising device is connected to the other end of the third pipe  63 . 
     The fan casing  26 , the first pipe  61 , the second pipe  62 , the steam suction ejector  44 , the third pipe  63  and the steam temperature-raising device  50  form an external circulation passage  60 . One end of a discharge passage  64  is connected to a discharge port  27  provided in a lower portion of the lateral side of the heating chamber  20 , and the other end of the discharge passage  64  is connected to one end of the exhaust gas duct  65 . The other end of the exhaust gas duct  65  is provided with an exhaust gas outlet  66 . A radiator  69  is outwardly fitted to the exhaust gas duct  65  side of the discharge passage  64 . A connection portion of the first pipe  61  and the second pipe  62  is connected to the exhaust gas duct  65  through an exhaust gas passage  67 . At the connection side with the first and second pipes  61 ,  62 , the exhaust gas passage  67  is provided with a damper  68  that opens/closes the exhaust gas passage  67 . 
     The steam temperature-raising device  50  includes a tray-shaped case  51  placed with its opening downward on a ceiling side at a central portion of the heating chamber  20 , a first steam superheater  52  placed in the tray-shaped case  51 , and a second steam superheater  53  placed in the tray-shaped case  51 . A bottom of the tray-shaped case  51  is formed of a metallic ceiling panel  54  serving as a ceiling plane of the heating chamber  20 . The ceiling panel  54  is formed with a plurality of ceiling steam outlets  55 . Both upper and lower surfaces of the ceiling panel  54  give a dark color by coating and the like. The ceiling panel  54  may also be formed of a metal material that turns into a dark color by repetitive use or a dark ceramic molded product. 
     One end of each of steam supply passages  23  (in  FIG. 3 , only one of them is shown), which extend to the left and right sides of the heating chamber  20 , is individually connected to the steam temperature-raising device  50 . The other end of each of the steam supply passages  23  extends downward along the side surfaces of the  5  heating chamber  20  and is connected to side steam outlets  22 , which are provided in a lower position of the lateral side of the heating chamber  20 , 
     Next, the steam generator  40  will be described in detail with reference to  FIGS. 4 ,  5 . 
     First,  FIG. 4(   a ) is a plan view, seen from above, of the pot  41  of the steam generator  40 , and  FIG. 4(   b ) is a side view of the pot  41 . 
     As shown in  FIGS. 4(   a ), ( b ), the pot  41  includes a cylinder portion  41   a  which has, in its horizontal plan view, a roughly rectangular shape, a bottom portion  41   b  provided on the lower side of the cylinder portion  41   a  and formed of an inclined plane that is gradually lowered toward a center portion of the bottom portion, and a water supply port  41   c  provided at a roughly center portion of the bottom portion  41   b.  Although the vertical to horizontal ratio of the planar shape of the pot  41  is 1:2.5, what is required of the planar shape is that the planar shape is an elongated shape, namely a rectangular or elliptic shape. Preferably, however, the vertical to horizontal ratio in the case of the rectangular shape is 1:2, more preferably 1:2.5, and most preferably 1:3 or less. 
     The heater  42  is placed in the vicinity of the bottom portion  41   b  in the pot  41 . The heater  42  is constructed of a first steam generation heater  42 A that is a U-shaped sheath heater having a larger pipe diameter and a second steam generation heater  42 B that is a U-shaped sheath heater having a smaller pipe diameter, which is placed inside the first steam generation heater  42 A and roughly on the same plane as the first generation heater  42 A. The heater  42  is placed adjacent to sidewalls of the cylinder portion  41   a  of the pot  41 . A minimum distance between an outer edge of the heater  42  and the sidewalls of the cylinder portion  41   a  is set to 2 mm to 5 mm. A lowermost portion of the heater  42  is placed adjacent to the bottom portion  41   b  of the pot  41 . A minimum distance between a lowermost portion of the heater  42  and the bottom portion  41   b  of the pot  41  is set to 2 mm to 5 mm. 
     In this embodiment, a 700 W sheath heater having a larger pipe size is used for the first steam generation heater  42 A, while a 300 W sheath heater having a smaller pipe size is used for the second steam generation heater  42 B. The first steam generation heater  42 A has a curved portion  42 Aa that has a roughly semicircular arc shape, and two linear portions  42 Ab,  42 Ac that extend roughly parallel from both ends of the curved portion  42 Aa. The second steam generation heater  42 B has a curved portion  42 Ba that has a generally semicircular arc shape, and two linear portions  42 Bb,  42 Bc that extend roughly parallel from both ends of the curved portion  42 Ba. The curved portion  42 Aa of the first steam generation heater  42 A has a minimum curvature radius r 1 , which is determined by the sheath heater having a larger pipe size to be used, while the curved portion  42 Ba of the second steam generation heater  42 B has a minimum curvature radius r 2  (&lt;r 1 ), which is determined by the sheath heater having a smaller pipe size to be used. 
     A water level sensor  43  is placed in the vicinity of the upper side of the heater  42  in the pot  41  and at a sidewall on the side of a non-heating portion (region C of  FIG. 4(   a )) inside the second steam generation heater  42 B. In the pot  41 , a partition plate  47  having a square cornered U-shape in cross section, which surrounds the water level sensor  43 , is provided. The partition wall  47  and the sidewall in the pot  41  form a casing having a rectangular shape in cross section. A lower end of the partition plate  47  is located on the upper side of the bottom portion  41   b  of the pot  41  and beneath the lowermost portions of the first and second steam generation heaters  42 A,  42 B. On the other hand, an upper end of the partition plate  47  is set at a level that is twice or more of a height from the lowermost portion of the heater  42  to an attaching position of the water level sensor  43 . Further, a temperature sensor  48  is placed at a sidewall opposite to the water level sensor  43  in the pot  41 . 
     The water level sensor  43  is a self-heating thermistor. In water, a temperature ranging from about 100° C. to 140° C. is detected depending on a water temperature ranging from about 20° C. to 100° C., while, in air, a temperature ranging from about 140° C. to 150° C. is detected. Based on the water temperature detected by the temperature sensor  48 , the temperature detected by the water level sensor  43  is determined, whereby the presence or absence of water, namely whether or not water is present at the attaching position of the water level sensor  43  is determined. 
       FIG. 5(   a ) is a side view of the steam generator  40 , and  FIG. 5(   b ) is a cross sectional view taken along line V-V of  FIG. 5(   a ). 
     As shown in  FIGS. 5(   a ), ( b ), the steam suction ejector  44  is attached in a manner so as to cover an upper side opening of the pot  41 , which is internally provided with the first and second steam generation heaters  42 A,  42 B. A fluid (steam) flowing in from an inlet  45   a  of the inner nozzle  45  is discharged from the opening  45   b  of the inner nozzle  45  and then discharged from the opening  46   a  of the outer nozzle  46 . At this time, since the discharge side of the inner nozzle  45  communicates with an internal space of the pot  41 , saturated steam generated in the pot  41 , which is led to the opening  46   a  side of the outer nozzle  46 , is discharged from the opening  46   a  of the outer nozzle  46  together with steam discharged from the opening  45   b  of the inner nozzle  45 . That is, saturated steam with a temperature of 100° C. and a pressure of 1 atm. (i.e., 1013.25 hPa), which is generated by boiling water in the pot  41 , is sucked into a circulating airflow that passes the external circulation passage  60  (shown in  FIG. 3) . By the structure of the steam suction ejector  44 , saturated steam is immediately sucked up. Since no pressure is applied in the steam generator  40 , discharge of saturated steam is not hindered. 
     Next, a control block diagram of the steam cooker  1  shown in  FIG. 6  will be described. 
     As shown in  FIG. 6 , connected to the control unit  80  are the blower fan  28 , the first steam superheater  52 , the second steam superheater  53 , the damper  68 , the drain valve  70 , the first steam generation heater  42 A, the second steam generation heater  42 B, the operation panel  11 , the water level sensor  36  for the water tank, the water level sensor  43 , a temperature sensor  81  for detecting a temperature in the heating chamber  20  (shown in  FIG. 3 ), a humidity sensor  82  for detecting a humidity in the heating chamber  20 , and the pump  35 . 
     The control unit  80  has a microcomputer and an input/output circuit. Based on detection signals received from the water level sensor  36  for the water tank, the water level sensor  43 , the temperature sensor  81 , and the humidity sensor  82 , the control unit  80  controls the blower fan  28 , the first steam superheater  52 , the second steam superheater  53 , the damper  68 , the drain valve  70 , the first steam generation heater  42 A, the second steam generation heater  42 B, the operation panel  11 , and the pump  35  in accordance with a predetermined program. 
     In the steam cooker  1  with the above construction, a power switch (not shown) on the operation panel  11  is pushed to power up, and a cooking operation is started by an operation of the operation panel  11 . Then, the control unit  80  first closes the drain valve  70 , and starts to operate the pump  35  in a state in which the exhaust gas passage  67  is closed by the damper  68 . Water is supplied by the pump  35  from the water tank  30  into the pot  41  of the steam generator  40  via the first through fourth water supply pipes  31 - 34 . Then, when the water level sensor  43  detects that the water level in the pot  41  has reached a predetermined level, the control unit  80  stops the pump  35  to stop water supply. 
     Next, the first and second steam generation heaters  42 A,  42 B are turned on, and a predetermined amount of water stored in the pot  41  is heated by the first and second steam generation heaters  42 A,  42 B. 
     Subsequently, simultaneously with turn-on of the first and second steam generation heaters  42 A,  42 B, or when the water temperature in the pot  41  has reached a predetermined temperature, the blower fan  28  is turned on, and, at the same time, the first steam superheater  52  is turned on. Then, the blower fan  28  sucks air including steam) in the heating chamber  20  through the intake openings  25  and sends the air (including steam) to the external circulation passage  60 . Since a centrifugal fan is used as the blower fan  28 , it is possible to generate a higher pressure than with a propeller fan. Furthermore, the centrifugal fan used as the blower fan  28  is rotated at a high speed by a DC motor, whereby a flow rate of the circulating airflow can be highly increased. 
     Next, when water in the pot  41  of the steam generator  40  boils, saturated steam is generated. The generated saturated steam merges into the circulating airflow that goes through the external circulation passage  60 , at the steam suction ejector  44 . Steam ejected from the steam suction ejector  44  flows into the steam temperature-raising device  50  at a high speed via the third pipe  63 . 
     Steam flowing into the steam temperature-raising device  50  is heated by the first steam superheater  52  to be superheated steam of about 300° C. (which varies depending on the type of cooking). A part of the superheated steam is jetted downward into the heating chamber  20  from the plurality of ceiling steam outlets  55  provided in the lower ceiling panel  54 . Another part of the superheated steam is jetted from the side steam outlets  22  in both of the lateral sides of the heating chamber  20  via the steam supply passages  23 , which extend to the left and right sides of the steam temperature heating device  50 . 
     Thereby, superheated steam jetted from the ceiling side of the heating chamber  20  is vigorously supplied to the object  90  side. Also, superheated steam jetted from the left and right lateral sides of the heating chamber  20  is supplied in a manner so as to wrap the object  90 , while going up from the lower side of the object  90 , after colliding with the catch pan  21 . Thereby, a convective superheated steam flow, in which steam blows down into a central portion and ascends in its outer side, is generated in the heating chamber  20 . The convective superheated steam flow repeats a cycle of being sucked into the intake openings  25 , passing the external circulation passage  60  and returning to the inside of the heating chamber  20  again. 
     In this manner, by forming a convective superheated steam flow in the heating chamber  20 , it becomes possible to jet superheated steam, which is sent from the steam temperature-raising device  50 , from the ceiling steam outlets  55  and the side steam outlets  22  so that it efficiently collides with the object  90 , while maintaining uniform temperature, humidity distribution in the heating chamber  20 . Then, collision of superheated steam heats the object  90 . At this time, superheated steam in contact with a surface of the object  90  heats the object  90  also by releasing latent heat that is generated when building up condensation on the surface of the object  90 . Thereby, a large quantity of heat can uniformly be imparted to the entire surface of the object  90  surely and promptly. Therefore, uniform cooking that secures a good finish can be realized. 
     In the cooking operation, an amount of steam in the heating chamber  20  increases with a lapse of time, and an excess amount of steam is discharged from the exhaust gas outlet  66  via the discharge port  27 , the discharge passage  64  and the exhaust gas duct  65 . At this time, a radiator  69  provided on the discharge passage  64  cools steam passing the discharge passage  64  to form condensation, whereby steam is prevented from being discharged to the outside as such. Water condensed by the radiator  69  within the discharge passage  64  runs down the discharge passage  64 , is led to the catch pan  21 , and disposed of together with water generated by cooking after completion of the cooking. 
     After completion of the cooking, the control unit  80  displays a message of completion of the cooking on the operation panel  11 , and a sound is raised by a buzzer (not shown) provided on the operation panel  11 . Thereby, a user who has noticed completion of the cooking opens the door  12 . Then, the control unit  80  opens the damper  68  immediately after a sensor (not shown) detects opening of the door  12 . Thereby, the first pipe  61  of the external circulation passage  60  communicates with the exhaust gas duct  65  via the exhaust gas passage  67 , so that steam in the heating chamber  20  is discharged by the fan  28  from the exhaust gas outlet  66  via the intake openings  25 , the first pipe  61 , the exhaust gas passage  67  and the exhaust gas duct  65 . The operation of the damper is the same even if the user opens the door  12  while cooking Therefore, the user can safely take out the object  90  from the inside of the heating chamber  20  without being exposed to steam. 
     The heater  42  is placed in the vicinity of the bottom portion  41   b  in the pot  41  of the steam generator  40  as well as roughly on the same level, whereby the water level of water supplied into the pot  41  can be set to a range from the bottom portion  41   b  of the pot  41  to a little above the upper side of the heater  42 . Therefore, by setting the water level in the pot  41  to a range from the bottom portion  41   b  of the pot  41  to a little above the upper side of the heater  42 , and making the water level as low as possible, the amount of water in the pot  41 , which is heated by the heater  42 , can be made as small as possible, and start of steam generation by the steam generator  40  can be quickened. By quickening the start of steam generation by the steam generator  40 , the start of superheated steam generation can be quickened, so that time required for cooking can be reduced. In particular, in cooking for the first time after stopping the operation for a long time, start of generation of superheated steam supplied into the heating chamber  20  can be quickened without requiring preheating while it is stopped. Thus, an effect of reducing the cooking time is noticeable. 
     The heater  42  is placed in the pot  41  whose planar shape is an elongated shape (a roughly rectangular shape in this embodiment), and the sheath heaters ( 42 A,  42 B) used as the heater  42  are placed in a manner so as to be along the sidewalls of the pot  41 , whereby an area taken up by a region surrounded by an outer edge of the heater  42  is reduced, thus making it possible to increase heater power relative to a floor area (or an area at the water level) taken up by the heater in the pot  41 , as well as possible to reduce a planar area of the pot  41 . Therefore, by increasing the heater power relative to the floor area (or the area at the water level) taken up by the heater in the pot  41 , and reducing the planar area of the pot  41  so that the amount of water is reduced, the steam generation by the steam generator  40  can be quickened more. 
     In the first steam generation heater  42 A that is a U-shaped sheath heater having a larger pipe size and the second steam generation heater  42 B that is a U-shaped sheath heater having a smaller pipe size, which is placed inside the first generation heater  42 A and roughly at the same level as the first generation heater  42 A, the radius of curvature of the curved portion  42 Ba is made to be the minimum. The radius of curvature is determined by the pipe size of the sheath heater and the like Then, if the power is applied to the heater in the same condition, the area taken up by the region surrounded by the outer edge of the heater  42  can be reduced so that the heater power relative to the area (or the area at the water level) taken up by the heater in the pot  41  is made highest. By making the heater power relative to the area (or the area at the water level) taken up by the heater in the pot  41  high, start of steam generation by the steam generator can be quickened more. Energization of the first steam generation heater  42 A having a high power (700 W) and energization of the second steam generation heater  42 B having a low power (300 W) is switched by the control unit  80 , thereby making it possible to control the power applied for steam generation depending on a combination of the heaters. This enables the generation of steam depending on the type of cooking. 
       FIG. 7(   a ) is a plan view, seen from below, of the steam temperature-raising device  50  of the steam cooker, and  FIG. 7(   b ) is a side view of the steam temperature-raising device seen from the side of steam supply ports. In the steam temperature-raising device  50 , as shown in  FIGS. 7(   a ), ( b ), the first steam superheater  52  that is a high power (1000 W) sheath heater having a larger pipe size and the second steam superheater  53  that is a low power (300 W) sheath heater having a smaller pipe diameter, or size are placed in the tray-shaped case  51  having a recessed part  51   a  whose planar shape is a roughly pentagonal shape. The second steam superheater  53  has a higher power density per unit surface area than that of the first steam superheater  52 . Although not shown in  FIGS. 7(   a ), ( b ), the opening of the recessed part  51   a  of the tray-shaped case  51  is covered by the metallic ceiling panel  54  (shown in  FIG. 3)  provided on the ceiling plane of the heating chamber  20 . 
     The recessed part  51   a  of the tray-shaped case  51  has a first sidewall  91  to which steam supply pipes  94 A,  94 B,  94 C are connected, a second sidewall  92 A which is adjacent to one side of the first sidewall  91  through a round portion  105 A and is roughly perpendicular to the first sidewall  91 , a third sidewall  92 B which is adjacent to the other side of the first sidewall  91  through a round portion  105 B and is roughly perpendicular to the first sidewall  91  and parallel to the second sidewall  92 A, a fourth sidewall  93 A which is adjacent to the second sidewall  92 A through a round portion  106 A and forms a blunt angle with the second sidewall  92 A, and a fifth sidewall  93 B which is adjacent to the third sidewall  92 B through a round portion  106 B, forms a blunt angle with the third sidewall  92 B, and also forms a blunt angle with the fourth sidewall  93 A. The fourth sidewall  93 A and the fifth sidewall  93 B are adjacent to each other through a round portion  107 . In the tray-shaped case  51 , the round portions  105 A,  106 A,  106 B,  107  that are corner portions of the recessed part  51   a  having a curved surface are formed by drawing. 
     In the tray-shaped case  51 , the side of the first sidewall  91  to which the steam supply pipes  94 A,  94 B,  94 C are connected corresponds to the back side ( FIG. 7(   a ) on the right) of the steam cooker  1 , while the side of the forth sidewall  93 A and the fifth sidewall  93 B corresponds to the front side ( FIG. 7(   a ) on the left) of the steam cooker  1 n The steam supply pipe  94 A having a steam supply port  95 A is connected to a roughly central portion of the first sidewall  91 , and the steam supply pipes  948 ,  94 C having steam supply ports  95 B,  95 C are connected on both sides of the steam supply pipe  94 A at predetermined intervals. Steam outlets  101 A,  102 A,  103 A,  104 A are provided at predetermined intervals in the second sidewall  92 A from the front side toward the back side, and at positions in the third sidewall  92 B opposite to the steam outlets  101 A- 104 A, steam outlets  101 B,  102 B,  103 B,  104 B are provided. The steam outlets  101 A- 104 A, and the steam outlets  101 B- 104 B are connected to the steam supply passage  23  shown in  FIG. 3 . The discharge side of the steam suction ejector  44  is connected to the inlet port side of the steam supply pipes  94 A,  948 ,  94 C via the third pipe  63  shown in  FIG. 3 . 
     A first and a second non-heating portion  52   b ,  52   c  of the first steam superheater  52  pass through the sidewall  91  and fixed at the outside of the steam supply pipes  94 A,  94 C. Electric wirings (not shown) are connected to leading ends of the first and the second non-heating portion  52   b ,  52   c  of the first steam superheater  52 . The first steam superheater  52  has a planar shape roughly symmetrical with respect to the center line L of the flow of steam flowing in from the steam supply ports  95 A,  95 B,  95 C, and includes the two non-heating portions  52   b ,  52   c  placed parallel to the center line L at a predetermined interval, two roughly U-shaped heating portions  52   a - 1 ,  52   a - 2  which are connected, at one end thereof, to the leading ends of the non-heating portions  52   b ,  52   c  and which are each curved toward the center of the recessed part  51   a , and a roughly U-shaped third heating portion  52   a - 3  which connects the two first and second heating portions  52   a - 1 ,  52   a - 2 . Spiral heat radiation fins  56  are provided around the first to third heating portions  52   a - 1  to  52   a - 3  and a part of the first and second non-heating portions  52   b ,  52   c  of the first steam superheater  52 . 
     Non-heating portions  53   b ,  53   c  at both ends of the second steam superheater  53  pass through the sidewall  91  and are fixed between the steam supply pipes  94 A and  94 B and between the steam supply pipes  94 B and  94 C, respectively. Electric wirings (not shown) are connected to leading ends of the non-heating portions  53   b ,  53   c  of the second steam superheater  53 . A heating portion  53   a  of the second steam superheater  53  has a circular shape and both ends of the circular heating portion  53  have a shape continuous with the non-heating portions  53   b ,  53   c . The second steam superheater  53  has a planar shape roughly symmetrical with respect to the center line L of the flow of steam flowing in from the steam supply ports  95 A,  95 B, and  95 C. 
     A part of steam in the recessed part  51   a  of the tray-shaped case  51  blows out into the heating chamber  20  from the steam outlets  101 A- 104 A,  101 B- 104 B, which are provided in the second and third sidewalls  92 A,  92 B facing each other across the center line L and on the side (front side) opposite from the steam supply ports  95 A,  95 B,  95 C, and also from the side steam outlets  22 A,  22 B (shown in  FIG. 12 ) via the steam supply passage  23  (shown in  FIG. 3 ). 
       FIG. 8(   a ) shows a plan view, seen from below, of a steam temperature-raising device not having a second steam superheater, and  FIG. 8(   b ) shows a side view of the steam temperature-raising device seen from the side of steam supply ports. The steam temperature-raising device  150  has the same construction as that of the steam temperature-raising device shown in  FIGS. 7(   a ) and  7 ( b ) except for not having the second steam superheater and the shape of the first steam superheater, and thus corresponding components are denoted by the same numerals and their description is omitted. In  FIG. 8(   a ), the reference numeral  151  denotes a tray-shaped case not provided with a fitting place for a second steam superheater, the reference numeral  152  denotes a first steam superheater, the reference numerals  105 A,  105 B,  106 A,  106 B, and  107  denote round portions. 
     Next, the steam flow at the steam temperature-raising device  50  will be described using  FIG. 9(   a ). 
     As shown in  FIG. 9(   a ), steam supplied from the steam generator  40  (shown in  FIG. 3)  passes the steam supply ports  95 A,  95 B,  95 C on the upper side in the figure (the back side) and flows downward in the figure (the front side) into the tray-shaped case  51 . For example, if focusing on a steam flow R 1  flowing in from the steam supply port  95 B, the steam flow R 1  is divided into a fourth sidewall  93 A side and a fifth sidewall  933  side to be branch flows R 2 A and R 2 B, respectively, in the vicinity of the round portion  107  of the recessed part  51   a  of the tray-shaped case  51 . A part of the branch flow R 2 A flows sideways along the fourth sidewall  93 A, turns in the vicinity of the round portion  106 A so as to flow along the second sidewall  92 A, and then turns inward in the round portion  105 A to merge into the flow R 1  again. Similarly, a part of the branch flow R 2 B flows sideways along the fifth sidewall  93 B, turns in the vicinity of the round portion  106 B so as to flow along the third sidewall  92 B, and turns inward in the round portion  105 B to merge into the flow R 1  again Further, steam flowing in from the other steam supply ports  95 A,  95 C also flows along the steam flow R 1  flowing in from the steam supply port  95 B. 
     In this manner, superheated steam is generated by heating steam in the recessed part  51   a  of the tray-shaped case  51  by means of the first and second steam superheaters  52 ,  53 . The thus generated superheated steam blows out from the ceiling steam outlets  55  (shown in  FIG. 3 ) and the steam outlets  101 A- 104 A,  101 B- 104 B. 
     In the steam temperature-raising device  50  of the above embodiment, the planar shape of the recessed part  51   a  of the tray-shaped case  51  is made in a roughly pentagonal shape having three sides forming a U-shape, and two sides that form a V-shape projecting outward, which are is continuous with both ends of the U-shape of the three sides. However, the planar shape of the recessed part of the tray-shaped case is not limited to this. For example, as shown in  FIG. 9(   b ), a shape having three sides forming a U-shape and a wall surface  96  with an arc-shaped outer periphery that is continuous with both ends of the U-shape of the three sides may be applied. 
     Contrary to this, for example, in the case where the tray-shaped case of the steam temperature-raising device has a shape in which front corner portions on both sides thereof have an acute angle as shown in  FIG. 10 , steam flows into a tray-shaped case  251  from the steam supply ports  95 A,  95 B,  95 C on the upper side of the figure (the back side) to the lower side of the figure (the front side) For example, if focusing on a steam flow R 11  that flows in from the steam supply port  95 B, the steam flow R 11  is divided on the opposite side of the steam supply port  95 B to be branch flows R 12 A and R 12 B. At this time, steam accumulations SA, SB are formed at the acute-angled corner portions, and due to the steam accumulations SA, SB, a steam temperature distribution in the tray-shaped case  251  becomes non-uniform. 
       FIG. 11(   a ) shows a view for describing heat radiation fins of the steam temperature-raising device of the steam cooker of the invention. This shows parts of the first and second heating portions  52   a - 1 ,  52   a - 2 , which are parallel to each other, of the first steam superheater  52  in the vicinity of the first sidewall  91  of the tray-shaped case  51  (shown in  FIGS. 7(   a ), ( b )). Since the spiral heat radiation fins  56  are provided in the same winding direction around the first steam superheater  52 , inclined planes of the heat radiation fins  56  are inclined in the same direction. Therefore, for example, influence of the inclined planes of the heat radiation fins  56  on steam flowing from the upper side to the lower side in the figure through the center of the first and second heating portions  52   a -  1 ,  52   a - 2 , which are parallel to each other, of the first steam superheater  52  shown in  FIG. 11(   a ) varies. However, in this embodiment, the fin pitch of the spiral heat radiation fins  56  that are provided around the first steam superheater  52  is set to 10 mm, whereby the distribution of the degree of influence (disturbance or resistance loss) of the heat radiation fins  56  on the steam flow blowing out from the steam supply ports  94 A,  94 B,  94 C (shown in  FIG. 9)  is made symmetrical with respect to the center line L of steam. 
     As shown in  FIG. 11(   b ), in the case where heat radiation fins  57  having a wider fin pitch than that of  FIG. 11(   a ) are provided around the first steam superheater  52 , the influence of the heat radiation fins  57  on the steam flow blowing out from the steam supply ports  94 A,  94 B,  95 C (shown in  FIG. 9)  varies between the left and right sides, so that the steam flow in the recessed part  51   a  of the tray-shaped case  51  is disturbed and not stabilized. Consequently, it becomes impossible to make the temperature distribution uniform. 
       FIG. 12  is a front elevational view of the steam cooker with its door opened. The intake openings  25  are placed in an upper corner of the back, and the circular panel  54  having the plurality of ceiling steam outlets  55  is attached to the center of the ceiling side of the heating chamber  20 . Further, the side steam outlets  22 A,  22 B, from which steam from the steam temperature-raising device  50  blows out, are placed in a lower position of the left and right lateral sides of the heating chamber  20 . The water tank  30  is placed on the observer&#39;s left of the heating chamber  20 . 
     In this manner, according to the steam cooker with the above construction, the planar shapes of the recessed part  51   a  of the tray-shaped case  51  and the first steam superheater  52  placed in the recessed part  51   a  thereof are made roughly symmetrical with respect to the center line L of the flow of steam flowing in from the steam supply ports  95 A,  95 B,  95 C, whereby steam flow in the recessed part  51   a  is not biased or deviated, the temperature distribution thereof is made uniform, and the temperature distribution of steam supplied into the heating chamber  20  from the ceiling steam outlets  55  of the ceiling panel  54  of the heating chamber  20  can be made uniform. 
     Further, the fin pitch of the spiral heat radiation fins  56 , which are wound around the first steam superheater  52  placed in the recessed part  51   a  of the tray-shaped case  51 , is set to 10 mm or less, whereby the degree of influence (disturbance or resistance loss) on the steam flow blowing out from the steam supply ports  95 A,  95 B,  95 C can be made roughly symmetrical with respect to the center line L. Consequently, the temperature distribution of steam in the recessed part  51   a  is made more uniform, and the temperature distribution of steam supplied into the heating chamber  20  from the ceiling steam outlets  55  of the ceiling panel  54  of the heating chamber  20  can be made uniform. Therefore, the temperature distribution in the heating chamber  20  can be made uniform, which makes it possible to perform uniform cooking. 
     By providing the steam outlets  101 A- 104 A,  101 B- 10   104 B in the second and the third sidewalls of the recessed part  51   a , respectively, which face each other across the center line L, and on the opposite side of the steam supply ports  95 A,  95 B,  95 C, and arranging the steam outlets  101 A- 104 A,  101 B- 104 B in the recessed part  51   a  so that they are roughly symmetrical with respect to the center line L, the amount, flow rate and temperature distribution of steam blowing sideways to the left or right from the inside of the recessed part  51   a  via the steam outlets  101 A- 104 A,  101 B- 104 B, can be made uniform. Thus, bias or unevenness in the temperature distribution of steam supplied sideways into the heating chamber  20  via the steam supply passage  23  (shown in  FIG. 3 ) can be reduced, so that the temperature distribution in the heating chamber  20  can be made more uniform. 
     Even if the planar shape of the recessed part of the tray-shaped case and the planar shape of the heater  50  are not symmetrical, by providing the plurality of steam outlets for supplying steam sideways into the heating chamber in both of the sidewalls adjacent to the sidewall provided with the steam supply ports of the recessed part of the tray-shaped case on the opposite side of the steam supply ports, the temperature distribution of steam blowing outward to both sides via the plurality of steam outlets can be made uniform. 
     Since the fourth and fifth sidewalls  93 A,  93 B of the recessed part  51   a  of the tray-shaped case  51  are inclined with respect to a plane roughly perpendicular to the center line L of the steam flow flowing in from the steam supply ports  95 A,  95 B,  95 C, when steam flowing into the recessed part  51   a  collides with the fourth and fifth sidewalls  93 A,  93 B to be divided into left and right side branches, the steam branch flows are made smooth, and generation of steam accumulation and the like causing uneven temperature distribution is suppressed, so that the temperature distribution of steam that flows in the recessed part  51   a  can be made uniform. 
     In the roughly pentagonal recessed part  51   a  of the tray-shaped case  51 , the two sides that are continuous with both ends of the U-shape of the three sides and are directed outward to form a V-shape serve as the fourth and fifth sidewalls  93 A,  93 B, which are inclined with respect to the plane roughly perpendicular to the center line L. Thereby, when steam flowing into the recessed part  51   a  of the tray-shaped case  51 , whose planar shape is symmetrical with respect to the center line L, collides with the fourth and fifth sidewalls  93 A,  94 B and is divided into the left and right side branches, the steam branch flows are made smooth, generation of steam accumulation and the like, which causes uneven temperature distribution, is suppressed, and the temperature distribution of steam flowing in the recessed part  51   a  can be made more uniform. 
     As shown in  FIG. 9(   b ), the planar shape of the recessed part  51   a  of the tray-shaped case  51  may be a shape having three sides forming a U-shape, and an arc-shaped outer periphery curved outward and connecting with both ends of the U-shape of the three sides. In this case, the arc-shaped outer periphery curved outward serves as the wall  96  inclined with respect to the plane perpendicular to the center line L. Thereby, when steam flowing into the recessed part  51   a  from the steam supply ports  95 A,  95 B,  95 C collides with the wall  96  having the arc-shaped outer periphery and is divided into the left and right side branches, the steam branch flows are made smooth, and generation of steam accumulation, which causes uneven temperature distribution, is suppressed, so that the temperature distribution of steam that flows in the recessed part  51   a  can be made more uniform. 
     Further, the round portions  105 A,  105 B,  106 A,  106 B,  107  having a curved surface are provided at the corners of the recessed part  51   a  of the tray-shaped case  51  so that adjacent sidewalls are smoothly continuous, whereby steam flowing into the recessed part  51   a  flows along the sidewalls of the recessed part  51   a  smoothly. Therefore, the steam flow in the recessed part  51   a  is stabilized, and the temperature distribution of steam flowing in the recessed part  51   a  is made more uniform. 
     As shown in  FIG. 7(   a ), because the heat radiation fins  56  are wound around the first to third heating portions  52   a - 1  to  52   a - 3  and also a part of the first and second non-heating portions  52   b ,  52   c , heat transferred from the side of the first, second heating portion  52   a - 1 ,  52   a - 2  to the side of the first, second non-heating portion  52   b,    52   c  of the first steam superheater  52  is radiated through the heat radiation fins  56  on the side of the non-heating portion  52   b ,  52   c.  Therefore, the first, second non-heating portion  52   b ,  52   c  is prevented from reaching a high temperature due to superheat. Thus, a temperature rise in the non-heating portion  52   b ,  52   c  of the first steam superheater  52 , to which electric wiring is connected, can be suppressed, and high reliability can be obtained. 
     Since the connection portions between the first and second heating portion  52   a - 1 ,  52   a - 2  to the first and second non-heating portion  52   b ,  52   c  of the first steam superheater  52  are located in positions where steam flowing in from the steam supply ports  95 A,  95 B,  95 C collides against the vicinity of the connection portions, steam having a lower temperature than that of steam superheated by the first steam superheater  52  is supplied through the steam supply ports  95 A,  95 C to the connection portions between the first and second heating portions  52   a - 1 ,  52   a - 2  and the first and second non-heating portions  52   b ,  52   c  of the first steam superheater  52 . Therefore, heat transferred from the first, second heating portions  52   a - 1 ,  52   a - 2  to the first and second non-heating portions  52   b ,  52   c  of the first steam superheater  52  can be radiated by the low-temperature steam through the heat radiation fins  56  on the side of the first and second non-heating portions  52   b,    52   c , so that a temperature rise in the non-heating portions  52   b ,  52   c , to which electric wiring is connected, can be suppressed effectively. Further, the connection portions of the first and second heating portions to the first and second non-heating portions of the first steam superheater may be placed at positions where steam flowing in from the steam supply ports directly collides against the connection portions. 
     Of the first steam superheater  52  having a high power and the second steam superheater  53  having a low power, the high-power first steam superheater  52  is provided with the heat radiation fins  56  around the first to third heating portions  52   a - 1  to  52   a - 3  and part of the non-heating portions  52   b ,  52   c  of, whereby heat transferred from the first, second heating portion  52   a - 1 ,  52   a - 2  with a high calorific power to the first, second non-heating portion  52   b ,  53   c  side of the first steam superheater  52  can be radiated through the heat radiation fins  56  on the side of the first, second non-heating portion  52   b ,  52   c , so that a temperature rise of the non-heating portion of the heater, to which electric wiring is connected, can be reduced more effectively. 
     Furthermore, the first and second steam superheaters  52 ,  53  are placed in the recessed part  51   a  of the tray-shaped case  51  of the steam temperature-raising device  50  such that the heat generation density per unit volume of a space in the vicinity of the steam supply ports  95 A,  95 B,  95 C is increased. Therefore, low-temperature steam flowing in from the steam supply ports  95 A,  95 B,  95 C collides with the first and second superheaters  52 ,  53 , which provide a region having a high heat generation density per unit volume, and the low-temperature steam is then heated, so that the temperature distribution in the entire recessed part  51   a  of the tray-shaped case  51  is made uniform. Thus, the temperature distribution of steam supplied from the ceiling steam outlets  55  into the heating chamber  20  can be made uniform Accordingly, the temperature distribution in the heating chamber  20  can be made uniform enabling even cooking. 
     Of the first and second superheaters  52 ,  53  having different power densities per unit surface area, the second superheater  53  having a higher power density per unit surface area is placed in the vicinity of the steam supply ports  95 A,  95 B,  95 C, whereby low-temperature steam flowing in from the steam supply ports  95 A,  95 B,  95 C is heated by the second steam superheater having the higher power density per unit surface area. Therefore, the start of steam temperature rise can be accelerated, so that a reduction in cooking time is ensured and the temperature distribution in the entire recessed part  51   a  of the tray-shaped case  51  can be made uniform more effectively. 
     Of the first and second superheaters  52 ,  53  with different power densities per unit surface area and having a planar shape that is symmetrical with respect to the center line L of the steam flow entering from the steam supply ports  95 A,  95 B,  95 C, the second steam superheater  53  having a high power density per unit surface area is placed inside, while the first steam superheater  52  having a low power density per unit surface area is placed outside, whereby low-temperature steam flowing in from the steam supply ports  95 A,  95 B,  95 C is heated by the second steam superheater  53  having a high power density per unit surface area, which is placed inside, and, in its outside, steam is heated by the first steam superheater  52  having a low power density per unit surface area. Therefore, the temperature distribution in the entire recessed part  51   a  of the tray-shaped case  51  can be made uniform more effectively. 
     Due to the use of the first and second steam superheaters  52 ,  53  having a planar shape that is symmetrical with respect to the center line L of the steam flow entering from the steam supply ports  95 A,  95 B,  95 C, steam flow is not biased toward the left or right side so that the temperature distribution in the recessed part  51   a  of the tray-shaped case  51  can be made uniform effectively. 
     The first steam superheater  52  has the two first and second non-heating portions  52   b ,  52   c  that are spaced from each other and placed roughly parallel to the center line L, the two roughly U-shaped first and second heating portions  52   a - 1 ,  52   a - 2  which are connected, at one end thereof, to leading ends of the first and second non-heating portions  52   b ,  52   c , respectively, and which are each curved toward the center of the recessed part  51   a  into a U shape, and the roughly U-shaped third heating portion  52   a - 3  that connects between the other ends of the first and second heating portions  52   a - 1 ,  52   a - 2 , and the first steam superheater  52  has a planar shape that is symmetrical with respect to the center line of the steam flow entering from the steam supply ports  95 A,  95 B,  95 C. Thus, steam flow in the recessed part  51   a  is not biased, so that the temperature distribution in the entire recessed part  51   a  of the tray-shaped case  51  can be made uniform effectively. 
     In the above embodiment, the heater  42  is used, which has the first steam generation heater  42 A that is a U-shaped sheath heater having a larger pipe size and the second steam generation heater  42 B that is a U-shaped sheath heater having a smaller pipe size, which is placed inside the first steam generation heater  42 A. The shape of the heater is not limited to this. However, the heaters are preferably placed roughly on the same level in the vicinity of the bottom portion in the pot. 
     In the above embodiment, the first steam superheater  52  (shown in  FIG. 7 ) as a planar heater is used for the steam temperature-raising device  50 . The shape of the heater is not limited to the above-described one, and any heater may be used as long as it has a planar shape that is roughly symmetrical with respect to the center line L of the steam flow flowing in from the steam supply ports. 
     For example, as shown in  FIG. 13 , a heater  58  placed in the recessed part  51   a  of the tray-shaped case  51  may be used. The heater  58  has a linear portion  58   a , a semi-circular curved portion  58   b , a linear portion  58   c , a semi-circular curved portion  58   d , a linear portion  58   e , a semi-circular curved portion  58   f , and a linear portion  58   g.    
     One end (on the side of the non-heating portion) of the linear portion  58   a  is inserted through the first side wall  91  of the tray-shaped case  51  in the vicinity of one end of the first sidewall  91 , and the other end of the linear portion  58   a  extends approximately parallel to the second sidewall  92 A up to the vicinity of the fourth sidewall  93 A. The other end of the linear portion  58   a  is then connected to one end of the semi-circular curved portion  58   b , and the other end of the semi-circular curved portion  58   b  is connected to one end of the linear portion  58   c , which extends approximately parallel to the linear portion  58   a  to the side of the first sidewall  91 . The other end of the linear portion  58   c  is connected to one end of the semi-circular curved portion  58   d , and the other end of the semi-circular curved portion  58   d  is connected to one end of the linear portion  58   e , which extends approximately parallel to the linear portion  58   c  to the opposite side of the first sidewall  91 . The other end of the linear portion  58   e  is connected to one end of the semi-circular curved portion  58   f , and the other end of the linear portion  58   f  is connected to one end of the linear portion  58   g , which extends approximately parallel to the third sidewall  92 B. The other end (on the side of the non-heating portion) of the linear portion  58   g  is inserted through the first sidewall  91  in the vicinity of the other end of the first sidewall  91 . The heater  58  has a planar shape that is roughly symmetrical with respect to the center line L of the steam flow entering from the steam supply ports  95 A,  95 B,  95 C.