Patent Publication Number: US-6216323-B1

Title: Buckle assembly

Description:
BACKGROUND OF THE INVENTION 
     a) Field of the Invention 
     This invention relates to a buckle assembly, and especially to a buckle assembly for a seat belt system arranged for a seat of a vehicle such as an automotive vehicle. 
     b) Description of the Related Art 
     Conventionally, a seat in a vehicle such as an automotive vehicle has been provided with a seal belt system. A buckle assembly for the seat belt system is generally provided with a tongue plate arranged on an end of a webbing and a buckle main body for releasably latching the tongue plate. Such buckle assemblies for seat belt systems include, for example, those disclosed in Pouget U.S. Pat. No. 4,182,008 and Clarke et al. U.S. Pat. No. 5,271,129. 
     A buckle main body of a buckle assembly disclosed in U.S. Pat. No. 4,182,008 is provided with a base, a slider arranged on the base such that the slider is pushed and moved by a tongue plate inserted in the base, a coil spring biasing the slider in a direction in which the tongue plate is pulled out, a latch plate movable to a tongue plate latching position when pushed by the slider, an auxiliary lock member, such as a pin, for retaining the latch plate in a latching state, a coil spring biasing the auxiliary lock member in the direction in which the tongue plate is pulled out, and a release button for releasing the latching state. 
     On the other hand, a buckle main body of a buckle assembly disclosed in U.S. Pat. No. 5,271,129 is provided with a base, a slider arranged on the base such that the slider is pushed and moved by a tongue plate inserted into the buckle assembly, a coil spring biasing the slider in a direction in which the tongue plate is pulled out, a latch plate movable to a tongue plate latching position when pushed by the slider, a block member slidable on the latch plate, a lock control member supported on the base, a coil spring biasing the lock control member, and a release button for releasing the latching state. 
     However, the buckle assembly disclosed in U.S. Pat. No. 4,182,008 involves a potential problem of occurrence of a so-called “false-latching phenomenon” in that, when performing a change-over of the latch plate between the latching position and an unlatching position by operating the release button, the auxiliary lock member may be held down by the base into a pinched state under the biasing force of the coil spring biasing the auxiliary lock member and the biasing force of the coil spring biasing the slider and may hence become stationary at an intermediate position. 
     The buckle assembly disclosed in U.S. Pat. No. 5,271,129 causes the block member, which plays the role of the auxiliary lock member, to move along a substantially L-shaped path, thereby making it more difficult to induce balancing of forces at an intermediate position where the latch plate may become stationary. However, this block member is also a force-bearing member. Formation of the block member with a metal to provide it with sufficient strength is accompanied by a problem of higher cost. If the block member is formed of a resin with a view to reducing its production cost, a potential problem then arises in that the reliability on its strength may be adversely affected when a high load or high impact is applied to the buckle assembly. 
     Buckle assemblies of another type include, for example, those disclosed in Barnes et al. U.S. Pat. No. 4,899,424 and Doty et al. U.S. Pat. No. 4,562,625. 
     A buckle assembly disclosed in U.S. Pat. No. 4,899,414 is provided with a base, a slider arranged on the base such that the slider is pushed and moved by a tongue plate inserted into the buckle assembly, a coil spring biasing the slider in a direction in which the tongue plate is pulled out, a latch plate movable to a tongue plate latching position when pushed by the slider, a lock member as a block member for retaining the latch plate in a latching state, a coil spring biasing the lock member in the direction in which the tongue plate is pulled out, and a release button equipped with a tilted surface for releasing the latching state and also for causing the lock member to move to a non-locking position. 
     Further, a buckle assembly disclosed in U.S. Pat. No. 4,562,625 is provided with a base, a slider arranged on the base such that the slider is pushed and moved by a tongue plate inserted into the buckle assembly, a lock member connected with the slider by an arm member and movable between a locking position and a non-locking position, a coil spring biasing the arm member, a latch plate movable to a latching position upon movement of the arm member and lock member when pressed by the slider, and a release button equipped with a tilted surface for causing the lock member to move to the non-locking position. 
     In each of the buckle assemblies disclosed in these patents, upon performing a change-over of the latch plate between the latching position and the unlatching position by operating the release button, a component of force is caused to occur in a non-locking direction (releasing direction) of the lock member by causing the lock member to move to the non-locking position with the tilted surface formed on the release button. 
     The buckle assemblies disclosed in of U.S. Pat. Nos. 4,899,424 and 4,562,625, however, may each develop an increase in the coefficient of friction between the tilted surface formed on the release button and the lock member or sticking between the above-described tilted surface and the lock member, for example, due to a foreign matter penetrated inside the buckle assembly. If such a situation arises, there is a potential problem that the division of force in the non-locking (release) direction by the tilted surface alone of the release button may become insufficient, leading to a potential risk that the tongue plate can be hardly released even when the release button is pressed. 
     SUMMARY OF THE INVENTION 
     The present invention has as an object the provision of a buckle assembly which can substantially eliminate the potential problem of a standstill of an auxiliary lock member at an intermediate position to reduce the occurrence of the false-latching phenomenon and which is provided with further enhanced reliability in strength. 
     The present invention also has as another object the provision of a buckle assembly which is provided with further enhanced reliability in the movement of a lock member to a non-locking position during a releasing operation in which the latching of a tongue plate by a latch plate is canceled to release the tongue plate from a buckle main body. 
     To achieve the former object, the present invention provides an buckle assembly comprising a tongue plate and a buckle main body for releasably holding said tongue plate. The buckle main body is provided with: 
     a base; 
     a lock control member supported on the base; 
     a latch plate supported on the base movably between a latching position, in which the latch plate is in latching engagement with the tongue plate, an unlatching position, in which the latch plate is out of latching engagement with the tongue plate; 
     an auxiliary lock member supported on the base movably between a retaining position, in which the auxiliary lock member retains the latch plate in the latching position, and a non-retaining position, in which the auxiliary lock member does not retain the latch plate in the latching position; and 
     a holder member held on the latch plate movably relative to the latch plate and provided with a cam portion for controlling a motion of the auxiliary lock member, said holder member being capable of assuming a first position, in which the holder member is in contact with the lock control member on a side of a leading edge of the lock control member as viewed in a direction of insertion of the tongue plate such that under biasing force of a biasing member, the latch plate is retained in the unlatching position and the auxiliary lock member is brought to the non-retaining position, and a second position, in which the holder member is located on a side of the lock control member, where the tongue plate to be inserted is positioned, such that the auxiliary lock member is brought to the retaining position. 
     In the buckle assembly constructed as described above, the auxiliary lock member receives a biasing force via the holder member when the tongue plate is brought into a latched state. On the other hand, upon bringing the tongue plate from the latched state into an unlatched state (released state), the holder member is first pressed and moved in a substantially horizontal direction by a pressing stroke of the release button, said pressing stroke being produced to cancel the latching by the latch plate, so that the auxiliary lock member is released from holding the latch plate. After that, the holder member changes the direction of its movement from the substantially horizontal direction to a substantially vertical direction, whereby the latch plate is caused to move to the unlatching position. At this time, the auxiliary lock member has already been released from its contact with the latch plate and owing to the cam portion of the holder member, the auxiliary lock member moves to a position where the auxiliary lock member controls the latch plate at the unlatching position. It is therefore possible to prevent the latch plate from becoming stationary at an intermediate position. 
     Described specifically, in the buckle assembly according to the present invention, the auxiliary lock member moves between the position, where the auxiliary lock member controls the latch plate at the latching position, and the unlatching position owing to the cam portion of the holder member. Further, the auxiliary lock member does not interfere with the latch plate except when the latch plate is in the fully locking position. It is therefore possible to avoid such a situation that the auxiliary lock member is balanced to become stationary at an intermediate position. As a consequence, the occurrence of the false-latching phenomenon can be prevented. 
     Further, a load which is applied to the buckle assembly can be borne by the latch plate, the auxiliary lock member and the buckle base, thereby making it possible to provide the buckle assembly with further enhanced reliability in strength. 
     To achieve the latter object of the present invention, the buckle assembly of the above-described construction may be constructed such that the buckle base is provided with a bottom wall and a pair of side walls extending upright from the bottom wall, and the buckle assembly is provided further with a release button for bringing the holder member from the second position to the first position when pressed and also with a control portion for controlling the release button at a leading end thereof, as viewed in a pressing direction of the release button, in a direction away from the bottom wall of the buckle base when a pressed stroke of the release button has increased. 
     According to the buckle assembly of the above-described construction, during a releasing operation in which the release button is pressed to cancel the latching of the tongue plate by the latch plate and hence to release the tongue plate form the buckle main body, the release button is guided by the control portion such that the holder member is caused to move to the first position. owing to this feature, even if a foreign matter or the like penetrates inside the buckle main body, it is still possible to obtain a sufficient component of force in a non-locking direction (release direction) of the holder member only by a pressing stroke of the release button produced during the releasing operation by the release button. This has made it possible to further enhance the reliability of movement of the holder member to the first position. 
     The control portion may be arranged in the form of slots bent or curved such that the release button can twist or prize the holder member toward the first position. More specifically, the slots may be formed in the side walls of the base, respectively, such that they are bent or curved toward upper parts of the side walls (i.e., in a direction away from the bottom wall of the base) on inner sides thereof as viewed in the pressing direction of the release button. 
     As another alternative, the control portion may also be formed of guide ridges bent or curved such that the release button can twist or prize the holder member toward the first position. 
     As a further alternative, the control portion may also be constructed such that it is bent from a point near a position, where the control portion begins to be pressed, to a point in the proximity of a position where the release button releases the holder member from the lock control member. 
     The arrangement of the control portion in any one of the above-described forms makes it possible to upset a balance between the release button and the holder member because a contact between the release button and the holder member becomes inconstant due to a guidance by the control portion in the course of the releasing operation by the release button. Further, the twisting or prizing of the holder member toward the first position by the release button can also bring about an advantageous effect in releasing sticking between the release button and the holder member. It is therefore possible to further enhance the reliability for the release of the tongue plate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an assembling or exploded view of a buckle assembly according to a first embodiment of the present invention; 
     FIG. 2 is a cross-sectional view of the buckle assembly according to the first embodiment in a stage of an operation; 
     FIG. 3 is a cross-sectional view of the buckle assembly according to the first embodiment in another of the operation; 
     FIG. 4 is a cross-sectional view of the buckle assembly according to the first embodiment in a further of the operation; 
     FIG. 5 is a cross-sectional view of the buckle assembly according to the first embodiment in a still further of the operation; 
     FIG. 6 is a cross-sectional view of the buckle assembly according to the first embodiment in a still further of the operation; 
     FIG. 7 is a cross-sectional view of the buckle assembly according to the first embodiment in a still further of the operation; 
     FIG. 8 is a cross-sectional view of the buckle assembly according to the first embodiment in a still further of the operation; 
     FIG. 9 is a cross-sectional view of the buckle assembly according to the first embodiment in a still further of the operation; 
     FIG. 10 is a cross-sectional view of the buckle assembly according to the first embodiment in a still further of the operation; 
     FIG. 11 is a cross-sectional view of the buckle assembly according to the first embodiment in a still further of the operation; 
     FIG. 12 is an assembling or exploded view of a buckle assembly according to a second embodiment of the present invention; 
     FIG. 13 is a cross-sectional view of the buckle assembly according to the second embodiment in a stage of an operation; 
     FIG. 14 is a cross-sectional view of the buckle assembly according to the second embodiment in another of the operation; 
     FIG. 15 is a cross-sectional view of the buckle assembly according to the second embodiment in a further of the operation; 
     FIG. 16 is a cross-sectional view of the buckle assembly according to the second embodiment in a still further of the operation; 
     FIG. 17 is a cross-sectional view of a buckle base as an element in the buckle assembly of the second embodiment; 
     FIG. 18 is a cross-sectional view of a buckle base as an element in a buckle assembly according to a first modification of the second embodiment of the present invention; and 
     FIG. 19 is a cross-sectional view of a buckle base as an element in a buckle assembly according to a second modification of the second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The buckle assemblies according to the first and second embodiments of the present invention will next gbe described with reference to the drawings. 
     As is illustrated in FIG.  1  through FIG. 11, the buckle assembly  1  according to the first embodiment of the present invention is provided with a tongue plate  5 , which is arranged at an end of an unillustrated webbing, and a buckle main body  10  for releasably latching the tongue plate  5 . 
     A tongue plate  5  is provided at an end portion thereof with a latching aperture  6  and at an opposite end portion thereof with a webbing slot  7  for connecting a webbing (not shown). 
     The buckle main body  10  is provided with a buckle base  11  functioning as a frame, a slider  12  movable in response to an insertion or removal of the tongue plate  5 , a coil spring  13  for causing the slider  12  to move, a latch plate  14  for latching the tongue plate  5 , a coil spring  15  for rocking the latch plate  14 , an auxiliary lock member  16 , preferably in the form of a pin, for controlling a rocking motion of the latch plate  14 , a holder member  17  for supporting the coil spring  15  and also movably supporting the auxiliary lock member  16 , a release button  18  for canceling latching of the tongue plate  5 , a coil spring  19  biasing the release button  18  in a direction in which the tongue plate  5  is pulled out, a spring holder  20  with which the coil spring  19  is maintained in contact, and a lock control member  21  for controlling an operation of the holder member  17 . 
     Incidentally, a plate  22  with an unillustrated webbing connected thereto like the tongue plate  5  is fixed by a rivet  23  on the buckle main body  10 . 
     The buckle base  11  is provided with a bottom wall  11 A and a pair of opposing side walls  11 B, 11 C arranged upright at opposite sides of the bottom wall  11 A, and is therefore configured substantially in a turned square U-shape as viewed in transverse cross-section. At an end portion of the bottom wall  11 A as viewed in a direction in which the tongue is pulled out (the direction indicated by arrow X in FIG.  2  through FIG. 11; hereinafter called “the direction of arrow X”), a guide floor  26  is formed such that the tongue plate  5  is guided by the guide floor  26  when the tongue plate  5  is inserted or removed. In addition, the bottom wall  11 A is also provided with an opening  25  for allowing the slider  12  to slide in a direction in which the tongue plate  5  is inserted and removed. At a substantially central part of an inner edge portion of the opening  25  as viewed in a direction in which the tongue  5  is inserted (the direction indicated by arrow Y in FIG.  2  through FIG. 11; hereinafter called “the direction of arrow Y”), a holding lug  45  is formed such that it holds the coil spring  13  at an end portion thereof as viewed in the direction of arrow Y. 
     At outer end portions of the side walls  11 B, 11 C as viewed in the direction of arrow X, engaged portions  29 B, 29 C are formed such that the spring holder  20  is fixedly secured. Below the engaged portions  29 B, 29 C and adjacent the bottom wall  11 A, rolled lugs  32 B, 32 C are formed extending inward. These rolled lugs  32 B, 32 C define a part of a mouth  53  through which the tongue plate  5  is inserted. 
     Through approximately central parts of the side walls  11 B, 11 C, horizontally-elongated slots  33 B, 33 C are formed as guide paths for supporting the auxiliary lock member  16  at opposite ends thereof movably in the directions of arrows X,Y (in other words, in a direction substantially parallel with the tongue inserting direction). Above outer edges of these slots  33 B, 33 C as viewed in the direction of arrow X, oval apertures  35 B, 35 C are formed to support the lock control member  21 . Further, on the side of inner edges of the slots  33 B, 33 C as viewed in the direction of arrow Y, receiving notches  36 B, 36 C are formed in the side walls  11 B, 11 C such that support portions  46 B, 46 C, which are formed at opposite side walls of an inner end portion of the latch plate  14  to be described in detail subsequently herein as viewed in the direction of arrow Y, are rockably inserted and supported. 
     The slider  12  is configured in a substantially turned U-shape as viewed in transverse cross-section, and is constructed such that the coil spring  13  is held at an end thereof in a recess  39  formed between a substantially planner upper plate  37  and a lower portion  38  arranged on a lower wall of the upper plate  37  and configured in an L-shape as viewed in longitudinal cross-section. Incidentally, the coil spring  13  is fixed at an opposite end thereof on the holding lug  45  of the bottom wall  11 A. 
     The slider  12  is slidably supported with the lower portion  38  thereof loose-fitted in the opening  25  of the buckle base  11 . When the tongue plate  5  is inserted into the buckle main body  10 , the slider  12  is pressed by the leading end of the tongue plate  5  and is hence caused to slide in the direction of arrow Y against biasing force of the coil spring  13 , whereby the slider  12  presses legs  44 B, 44 C, which are arranged on lower ends of both side portions of the latch plate  14  to be described in detail subsequently herein, so that the latch plate  14  is caused to pivot counterclockwise (in a direction in which the latch plate  14  latches the tongue plate  5 ). When the tongue plate  5  is pulled out, on the other hand, the slider  12  is caused to return in the direction of arrow X under the biasing force of the coil spring  13 . 
     Through an approximately central part of the latch plate  14 , an opening  40  is formed such that the holder member  17  to be described in detail subsequently herein is partly inserted there. At an outer end portion of the latch plate  14  as viewed in the direction of arrow X, a latch portion  41 —which is to be inserted into the latching aperture  6  of the tongue plate  5  for the latching of the tongue plate  5  when the tongue plate  5  is inserted into the buckle main body  10 —is formed extending downward. Formed between the latch portion  41  and the opening  40  is a support surface  42  which can be brought into contact with the auxiliary lock member  16  to be described in detail subsequently herein. On the opposite side walls of the inner end portion of the latch plate  14  as viewed in the direction of arrow Y, the support portions  46 A, 46 B are arranged extending from the opposite side walls. These support portions  46 A, 46 B are rockably (pivotally) supported in the receiving notches  36 B, 36 C of the side walls  11 B, 11 C. Described specifically, the latch plate  14  rocks about the support portions  46 B, 46 C as fulcrums and moves to the latching position, in which the latch plate  14  latches the tongue plate  5 , or to an unlatching position, in which the latching of the tongue plate  5  is cancelled. On the opposite side walls of an inner end portion of the latch plate  14  as viewed in the direction of arrow Y, the legs  44 B, 44 C are arranged extending downward. The slider  12  can be brought into contact with these legs  44 B, 44 C as mentioned above. 
     The holder member  17  is provided with a main part  51 , side walls  52 B, 52 C, which are formed on opposite side walls of the main part  51 , and a shaft  47  formed at a substantially central part of an outer end portion of the main part  51  as viewed in the direction of arrow Y. 
     The main part  51  is formed of a curved surface  54  arranged on an outer side as viewed in the direction of arrow X, a planar surface  55  formed in continuation with an upper part of the curved surface  54 , a tilted surface  56  arranged in continuation with the planar surface  55 , and a protuberance  58  extending downward from a substantially central part of the tilted surface  56 . 
     The protuberance  58  is inserted in the opening  40  of the latch plate  14 , and at end walls thereof as viewed in a transverse direction, is provided with claws  58 B, 58 C which project outward in the transverse direction. Both of these claws  58 B, 58 C are maintained in slidable engagement with the lower wall of the latch plate  14  at locations adjacent the opening  40 . Namely, the holder member  17  is arranged such that the latch plate  14  is embraced by both of the claws  58 B, 58 C. 
     Incidentally, the main part  51  is open at a surface (a lower surface in FIG. 1) located opposite the planar surface  55 , and is constructed such that the auxiliary lock member  16  inserted in the support apertures  57 B, 57 C to be described in detail subsequently herein are exposed through the lower surface. 
     Through substantially central parts of the side walls  52 B, 52 C, the support apertures  57 B, 57 C are opened as cam portions in the form of elongated apertures tilted relative to the planar surface  55 . Through these support apertures  57 B, 57 C, the auxiliary lock member  16  is movably inserted. Opposite end portions of the auxiliary lock member  16  which extends through the support apertures  57 B, 57 C are inserted in the slots  33 B, 33 C movably in the directions of arrows X,Y. 
     In a stage where the tongue plate  5  has not been inserted yet (see FIG.  2  and FIG.  11 ), the holder member  17  is biased in the direction of arrow X by the coil spring  15  and is maintained in contact with the lock control member  21  to be described in detail subsequently herein. The auxiliary lock member  16  is therefore located on the side of the inner edges of the support apertures  57 B, 57 C and the slots  33 B, 33 C as viewed in the direction of arrow Y. At this time, the support apertures  57 B, 57 C are arranged such that they are oblique relative to the slots  33 B, 33 C, in other words, they intersect at a predetermined angle the slots  33 B, 33 C, respectively. 
     The shaft  47  is formed such that it has smaller diameter at a free end thereof, which is an inner end as viewed in the direction of arrow Y, than at a basal end thereof which is an outer end as viewed in the direction of arrow X. The coil spring  15  is loose-fitted on the shaft  47  so that owing to the arrangement of the shaft  47 , the coil spring  15  is prevented from being bent or broken when it undergoes expansion and contraction. The small-diameter portion of the shaft  47  is dimensioned such that the small-diameter portion of the shaft  47  can freely extend back and forth through an opening  49  formed in an abutting portion  43  of the latch plate  14 . On the other hand, the large-diameter portion of the shaft  47  is dimensioned greater than the diameter of the opening  49 . 
     As has been described above, the shaft  47  freely extends back and forth through the opening  49  of the latch plate  14 , and the claws  58 B, 58 C are in slidable engagement with the latch plate  14  in such a way that the claws  58 B, 58 C embrace the latch plate  14  therein. In response to a movement of the latch plate  14 , the holder member  17  can therefore, for example, slide on the latch plate  14  in a direction substantially parallel with the tongue plate inserting direction in the state of FIG. 5 in which the latch plate  14  is in the locking position. 
     The lock control member  21  is constructed of a pin-like member having an oval shape in transverse cross-section, and is inserted and supported in the oval apertures  35 B, 35 C formed in the side walls  11 B, 11 C. Incidentally, this lock control member  21  is arranged at a position where the holder member  17  can be brought into contact with the lock control member  21 . 
     The spring holder  20  is provided at transversely opposite ends thereof with engaging portions  27 B, 27 C, which are brought into engagement with the engaged portions  29 B, 29 C formed on the side walls  11 B, 11 C, respectively. On a substantially central part of the spring holder  20 , a recessed spring seat  28  is formed to fixedly hold the coil spring  19  at an inner end portion thereof as viewed in the direction of arrow Y. Formed on a lower part of the spring holder  20  is an upper wall forming portion  31  as an upper wall of the mouth  53 . The engaging portions  27 B, 27 C are maintained in fixed engagement with the engaged portions  29 B, 29 C, so that the spring holder  20  is fixedly secured on the buckle base  11 . As a result of this fixed securement, the mouth  53  is formed by the upper wall forming portion  31 , the rolled lugs  32 B, 32 C and the guide floor  26 . 
     The release button  18  is provided with an operating portion  61 , which can be pressed by a user, and arms  62 B, 62 C arranged on opposite sides of the operating portion  61  such that the arms  62 B, 62 C extend out horizontally in the direction of arrow Y. This release button  18  moves in the directions of arrows X,Y with the arms  62 B, 62 C slidably supported by the side walls  11 B, 11 C. Further, on an inner end wall of the operating portion  61  of the release button  18  as viewed in the direction of arrow Y, fingers  63 B, 63 C are formed such that they can be brought into contact with the curved surface  54  of the holder member  17 . 
     A recessed spring seat  64  for holding the coil spring  19  is formed on the inner end wall of the operating portion  61  at a substantially central part thereof. As the coil spring  19 .held by the recessed spring seat  64  is fixedly held at the opposite end thereof by the recessed spring seat  28 , the release button  18  is biased normally in the direction of arrow X by the coil spring  19 . 
     The buckle main body  10  constructed as described above is enclosed in an unillustrated cover. 
     A description will next be made about a specific operation of the buckle assembly according to the first embodiment. 
     When the tongue plate  5  has not been inserted yet in the buckle main body  10 , the slider  12  is placed in the state that, as is illustrated in FIG.  2  and FIG.  11 , the slider  12  is in contact with the inner edge portion, as viewed in the direction of arrow X, of the opening  25  of the bottom wall  11 A of the buckle base  11  by the biasing force of the coil spring  13 . 
     The holder member  17  is biased in an upper left direction by the coil spring  15 , whereby the support apertures  57 B, 57 C in the holder member  17  and the slots  33 B, 33 C in the side walls  11 B, 11 C are arranged at a certain angle therebetween, respectively. 
     Further, the auxiliary lock member  16  is located at the inner edges of the support apertures  57 B, 57 C as viewed in the direction of arrow Y and also at the inner edges of the slots  33 B, 33 C (in the non-locking position). At this time, the latch plate  14  is allowed to retain under the biasing force of the coil spring  15  the state that the latch portion  41  is located in an upper position. Accordingly, the auxiliary lock member  16  is out of contact with the latch plate  14 . 
     To latch the tongue plate  5  in the buckle main body  10 , the tongue plate  5  is firstly inserted into the buckle main body  10  through the mouth  53  as illustrated in FIG.  2  and FIG.  3 . At this time, the leading end of the tongue plate  5  causes the slider  12  to move in the direction of arrow Y so that the coil spring  13  is compressed. When the slider  12  comes into contact with the legs  44 B, 44 C of the latch plate  14 , the latch plate  14  begins to pivot toward the locking position (in a counterclockwise direction shown in the drawings) about the support portions  46 B, 46 C as fulcrums. 
     Concurrently with the above motion, the holder member  17  begins to move in response to the pivotal motion of the latch plate  14  while compressing the coil spring  15  with the curved surface  54  maintained in contact with the lock control member  21 . Described specifically, the holder member  17  moves downward in a substantially vertical direction together with the latch plate  14  while being prevented from moving approximately in the direction of arrow X by the lock control member  21 . By this movement of the holder member  17 , the auxiliary lock member  16  is pressed by upper edges of the support apertures  57 B, 57 C as shown in FIG. 4, so that the auxiliary lock member  16  begins to move toward the retaining position in the direction of arrow X in the slots  33 B, 33 C of the side walls  11 B, 11 C. Namely, the auxiliary lock member  16  undergoes the above-described movement by the cam action of the support apertures  57 B, 57 C and that of the slots  33 B, 33 C. 
     When the tongue plate  5  is inserted, the latch plate  14  and the holder member  17  move further as illustrated in FIG.  5 . The curved surface  54  of the holder member  17  is then caused to move beyond the lock control member  21 , whereby the holder member  17  disengages toward a point below the lock control member  21  (toward a side on which the inserted tongue plate  5  is located). As a result, the holder member  17  becomes movable substantially in the direction of arrow X. 
     Subsequent to this motion, the planar surface  55  of the holder member  17  comes into contact with the lower surface of the lock control member  21  as illustrated in FIG. 6, and by the biasing force of the coil spring  15 , the holder member  17  changes the direction of its movement substantially in the direction of arrow X. As a consequence, the auxiliary lock member  16  moves at a stretch to the outer edges of the support apertures  57 B, 57 C of the holder member  17  as viewed in the direction of arrow X and also to the outer edges of the slots  33 B, 33 C as viewed in the direction of arrow X, that is, to the retaining position, so that the auxiliary lock member  16  is brought into contact with the support surface  42  of the latch plate  14 . By this time, the latch plate  14  has moved to the latching position and has completed the latching of the tongue plate  5 . It is therefore possible to prevent the auxiliary lock member  16  from being held between the latch plate  14  and the slots  33 B, 33 C of the side walls  11 B, 11 C of the buckle base  11 . 
     Since the auxiliary lock member  16  is allowed to come into contact with the support surface  42  of the latch plate  14  only when the latch plate  14  is in the latching position, it is possible to prevent the auxiliary lock member  16  from attaining a balance and becoming stationary at an intermediate position. 
     In this state, a force applied toward the unlatching position of the latch plate  14  (in the releasing direction) is borne by the support surface  42  of the latch plate  14 , the auxiliary lock member  16 , and the slots  33 B, 33 C of the side walls  11 B, 11 C. The force is therefore divided, thereby making it possible to further enhance the reliability for strength. 
     Next, upon pulling the tongue plate  5  out of the buckle main body  10 , the release button is pressed in the direction of arrow Y. The fingers  63 B, 63 C of the release button  18  then come into contact with the curved surface  54  of the holder member  17  as shown in FIG.  7 . 
     When the release button  18  is pressed further, the holder member  17  begins to move in the direction of arrow Y (in a substantially horizontal direction) as depicted in FIG.  8 . Accordingly, the auxiliary lock member  16  is pressed via the support apertures  57 B, 57 C of the holder member  17  and begins to move in the direction of arrow Y, whereby the contact between the auxiliary lock member  16  and the support surface  42  of the latch plate  14  is canceled. At this time point, the outer edge, as viewed in the direction of arrow X, of the planar surface  55  of the holder member  17  is still located on the lower surface of the lock control member  21 , so that the latch plate  14  is held by the holder member  17  and remains in the latching position. Further, the holder member  17  is also prevented from moving in a substantially vertical direction. 
     When the release button  18  is pressed still further, the outer edge, as viewed in the direction of arrow X, of the planar surface  55  of the holder member  17  is caused to move beyond the lock control member  21  and disengages from the lower surface of the lock control member  21  as shown in FIG.  9 . As a result, it becomes possible for the holder member  17  to move in a substantially vertical direction. In continuation with this, the holder member  17 , as is illustrated in FIG. 10, moves upward in a substantially vertical direction at a stretch by the biasing forces of the coil springs  15 , 13  with the curved surface  54  maintained in contact with the lock control member  21 . 
     Concurrently with this upward movement of the holder member  17  in the substantially vertical direction, the latch plate  14  pivots toward the unlatching position (in a clockwise direction) by the biasing force of the coil spring  15  about the support portions  46 B, 46 C as fulcrums and moves to the unlatching position, so that the latching of the tongue plate  5  is canceled as shown in FIG.  11 . 
     At the same time, the auxiliary lock member  16  is pressed by the lower edges of the support apertures  57 B, 57 C of the holder member  17  and moves in the direction of arrow Y at a stretch in the slots  33 B, 33 C of the side walls  11 B, 11 C. Described specifically, the auxiliary lock member  16  is caused to move by the cam action of the support apertures  57 B, 57 C and that of the slots  33 B, 33 C. At this time, the auxiliary lock member  16  is supported only in the support apertures  57 B, 57 C and the slots  33 B, 33 C and is out of contact with the latch plate  14 . The auxiliary lock member  16  therefore moves to a non-retaining position without stopping at an intermediate position. 
     As has been described above, the auxiliary lock member  16  does not interfere with the latch plate  14  except when the latch plate  14  is exactly in the locking position, and when the latch plate  14  is about to move toward the unlatching position, the auxiliary lock member  16  moves at a stretch by the above-mentioned cam actions. Upon performing a change-over of the latch plate  14  between the latching position and the unlatching position, the auxiliary lock member  16  is therefore held and pinched by the biasing force of the coil spring  15 , which urges the auxiliary lock member  16 , and the biasing force of the coil spring  13 , which urges the slider  12 , thereby making it possible to prevent the auxiliary lock member  16  from attaining a balance and becoming stationary at an intermediate position. 
     In the first embodiment described above, the support apertures  57 B, 57 C of the holder member  17  were formed as elongated apertures. It is however to be noted that the support apertures  57 B, 57 C are not limited to such a shape and can be apertures of a different shape insofar as cam action can be produced. 
     A description will next be made about the second embodiment of the present invention. 
     As is illustrated in FIG.  12  through FIG. 17, the buckle assembly  100  according to the second embodiment of the present invention is provided with a tongue plate  105 , through a webbing slot  106  of which an unillustrated webbing extends, and a buckle main body  110  for releasably latching the tongue plate  105 . 
     The tongue plate  105  is provided at an end portion thereof with a latching aperture  160  and at an opposite end portion thereof with the webbing slot  107  for permitting therethrough an insertion of the webbing (not shown). 
     The buckle main body  110  is provided with a buckle base  111  functioning as a frame, a slider  112  movable in response to an insertion or removal of the tongue plate  105 , a coil spring  113  for causing the slider  112  to move, a latch plate  114  for latching the tongue plate  105 , a coil spring  115  for rocking the latch plate  114 , a lock pin  116  for controlling a rocking motion of the latch plate  114 , a holder member  117  for supporting the coil spring  115  and also movably supporting the lock pin  116 , a release button  118  for canceling latching of the tongue plate  105 , a coil spring  119  biasing the release button  118  in a direction in which the tongue plate  105  is pulled out, a spring holder  120  with which the coil spring  119  is maintained in contact, a switch  121  for detecting a latched or unlatched state of the tongue plate  105 , and a lower cover  122 L and an upper cover  122 U between which the above-described elements are accommodated. 
     In the second embodiment, the lock member comprises the holder member  117  and the lock pin  116 . 
     Incidentally, a stay  107  is fixed at a free end thereof on the buckle main body  110  by a rivet  123 . 
     The buckle base ill is provided with a bottom wall  111 A and a pair of opposing side walls  111 B, 111 C arranged upright at opposite sides of the bottom wall  11 A, and is therefore configured substantially in a turned square U-shape as viewed in transverse cross-section. At an end portion of the bottom wall  111 A as viewed in a direction in which the tongue plate is pulled out (the leftward direction in FIG.  12  through FIG. 16; this direction will hereinafter be called “leftward”), a guide floor  126  is formed such that the tongue plate  105  is guided by the guide floor  126  when the tongue plate  105  is inserted or removed. In addition, the bottom wall  111 A is also provided with an opening  125  for allowing the slider  112  to slide in a direction in which the tongue plate  105  is inserted and removed. At a substantially central part of an inner edge portion of the opening  125  as viewed in a direction in which the tongue plate  105  is inserted (the rightward direction in FIG.  12  through FIG. 16; this direction will hereinafter be called “rightward”), a holding lug  145  is formed such that it holds the coil spring  113  at a right end portion thereof. 
     Through approximately central parts of the side walls  111 B, 111 C, horizontally-elongated slots  133 B, 133 C are formed such that the lock pin  116  is supported at opposite ends thereof movably leftward and rightward. Below the slots  133 B, 133 C, cranked slots  134 B, 134 C are formed as a control portion for movably supporting beads  129 A, 129 B of the release button  118  to be described subsequently herein. Each of these cranked slots  134 B, 134 C has such a substantially crank shape that it is bent upward on a right side of an approximately central part in the horizontal direction and that it again extends in a substantially horizontal direction. In other words, each of the cranked slots  134 B, 134 C is formed such that it is bent from a position where the release button  118  presses the holder member  117  to a point in the proximity of a position at which the holder member  117  is released from the lock control member  131  to be described in detail subsequently herein. 
     Further, on the right sides of the slots  133 B, 133 C, receiving notches  136 B, 136 C are formed such that support portions  146 B, 146 C, which are formed at opposite side walls of an inner end portion of the latch plate  114  to be described in detail subsequently herein, are rockably inserted and supported. 
     The slider  112  is configured in a substantially turned U-shape as viewed in transverse cross-section, and is constructed such that the coil spring  113  is held at a left end thereof in a recess  139  formed between a substantially planner upper plate  137  and a lower portion  138  arranged on a lower wall of the upper plate  137  and configured in an L-shape as viewed in longitudinal cross-section. Formed on opposite side walls of a right end portion of the slider  112  are pressing portions  135 B, 135 C for pressing legs  144 B, 144 C of the latch plate  114  to be described in detail subsequently herein. Incidentally, the coil spring  113  is fixed at a right end thereof on the holding lug  145  of the bottom wall  11 A. 
     The slider  112  is slidably supported with the lower portion  138  thereof loose-fitted in the opening  125  of the buckle base  111 . When the tongue plate  105  is inserted into the buckle main body  110 , the slider  112  is pressed by the leading end of the tongue plate  105  and is hence caused to slide rightward against biasing force of the coil spring  113 , whereby the slider  112  presses the legs  144 B, 144 C, which are arranged on lower ends of both side portions of the latch plate  114  to be described in detail subsequently herein, so that the latch plate  114  is caused to pivot counterclockwise (in a direction in which the latch plate  114  latches the tongue plate  105 ). When the tongue plate  105  is pulled out, on the other hand, the slider  112  is caused to return leftward under the biasing force of the coil spring  113 . 
     Through an approximately central part of the latch plate  114 , an opening  140  is formed such that the holder member  117  to be described in detail subsequently herein is partly inserted there. At a left end portion of the latch plate  114 , a latch portion  141 —which is to be inserted into the latching aperture  160  of the tongue plate  105  for the latching of the tongue plate  105  when the tongue plate  105  is inserted into the buckle main body  110 —is formed extending downward. Formed between the latch portion  141  and the opening  140  is a support surface  142  which can be brought into contact with the lock pin  116  to be described in detail subsequently herein. 
     On the opposite side walls of the right end portion of the latch plate  114 , the support portions  146 A, 146 B are arranged extending from the opposite side walls. These support portions  146 A, 146 B are rockably (pivotally) supported in the receiving notches  136 B, 136 C of the side walls  111 B, 111 C. Described specifically, the latch plate  114  rocks about the support portions  146 B, 146 C as fulcrums and moves to the latching position, in which the latch plate  114  latches the tongue plate  105 , or to an unlatching position, in which the latching of the tongue plate  105  is cancelled. On the opposite side walls of a right end portion of the latch plate  114 , the legs  144 B, 144 C are arranged extending downward. The slider  112  can be brought into contact at pressing portions  135 B, 135 C thereof with these legs  144 B, 144 C as mentioned above. 
     At a substantially central part of the right end portion of the latch plate  114 , an abutting portion  143  is formed upright. Formed through a substantially central part of the abutting portion  143  is an opening  149 . A shaft  147  to be described in detail subsequently herein can be inserted into the opening  149  and can also be pulled out of the opening  149 . 
     The holder member  117  is provided with a main part  151 , side walls  152 B, 152 C, which are formed on opposite side walls of the main part  151 , and the shaft  147  formed at a substantially central part of a right end portion of the main part  151 . 
     The main part  151  is formed of a curved surface  154  arranged on a left outer side, an upper surface  155  formed in continuation with an upper part of the curved surface  154 , a tilted surface  156  arranged in continuation with the upper surface  155 , and a protuberance  158  extending downward from a substantially central part of the tilted surface  156 . 
     Extending over the curved surface  154  and the upper surface  155 , two ridges  151 B, 151 C are formed such that they can be brought into contact with a face  132 A of the spring holder  120  to be described in detail subsequently herein. The face  132 A defines a left end of an opening  132  formed in a upper part of the spring holder  120 . 
     The protuberance  158  is inserted in the opening  140  of the latch plate  114 , and at end walls thereof as viewed in a transverse direction, is provided with claws  158 B, 158 C which project outward in the transverse direction. Both of these claws  158 B, 158 C are maintained in slidable engagement with the lower wall of the latch plate  114  at locations adjacent the opening  140 . Namely, the holder member  117  is arranged such that the latch plate  114  is embraced by both of the claws  158 B, 158 C. 
     Incidentally, the main part  151  is open at a surface (a lower surface) located opposite the upper surface  155 , and is constructed such that the lock pin  116  inserted in the support apertures  157 B, 157 C formed in the side walls  152 B, 152 C, said support apertures  157 B, 157 C being to be described in detail subsequently herein, are exposed through the lower surface. 
     Through substantially central parts of the side walls  152 B, 152 C, the support apertures  157 B, 157 C are opened in the form of elongated apertures tilted relative to the upper surface  155 . Through these support apertures  157 B, 157 C, the lock pin  116  is movably inserted. Opposite end portions of the lock pin  116  which extends through the support apertures  157 B, 157 C are inserted in the slots  133 B, 133 C of the buckle base  111  movably leftward and rightward. Formed on lower left end portions of the side walls  152 B, 152 C are abutting portions  159 B, 159 C, with which tilted portions  163 B, 163 C of the release button  118  can be brought into contact upon effecting a release of the tongue plate  105  by pressing the release button  118  as will be described in detail subsequently herein. 
     In a stage where the tongue plate  105  has not been inserted yet (see FIG.  13  and FIG.  16 ), the holder member  117  is biased leftward by the coil spring  115 . 
     The lock pin  116  is therefore located on the side of the right edges of the support apertures  157 B, 157 C and the slots  133 B, 133 C. At this time, the support apertures  157 B, 157 C are arranged in positions oblique relative to the slots  133 B, 133 C, in other words, at predetermined angles relative to the slots  133 B, 133 C. 
     The shaft  147  is formed such that it has smaller diameter at a right end thereof than at a left end thereof. The coil spring  115  is loose-fitted on the shaft  147  so that owing to the arrangement of the shaft  147 , the coil spring  115  is prevented from being bent or broken when it undergoes expansion and contraction. The small-diameter portion of the shaft  147  is dimensioned such that the small-diameter portion of the shaft  147  can freely extend back and forth through the opening  149  formed in the abutting portion  143  of the latch plate  114 . On the other hand, the large-diameter portion of the shaft  147  is dimensioned greater than the diameter of the opening  149 . 
     As has been described above, the shaft  147  freely extends back and forth through the opening  149  of the latch plate  114 , and the claws  158 B, 158 C are in slidable engagement with the latch plate  114  in such a way that the claws  158 B, 158 C embrace the latch plate  114  therein. The holder member  117  can therefore slide on the latch plate  114  in response to a movement of the latch plate  14 . 
     The spring holder  120  is arranged on the left side of the buckle base  111 . On a substantially central part of the left end wall of the spring holder  120 , a raised spring seat  128  is formed to fixedly hold the coil spring  119  at a right end portion thereof. Between a lower wall of the spring holder  120  and the bottom wall  11 A of the buckle base  111 , a space is formed as a mouth  153  to permit an insertion of the tongue plate  105 . 
     The opening  132  is formed in an upper part of the spring holder  120 . The opening  132  is constructed such that, when the tongue plate  105  has been brought into a latched state (see FIG.  14  and FIG.  15 ), the curved surface  154  and upper surface  155  of the holder member  117  penetrate under a portion which defines a left edge of the opening  32 . When the tongue plate  105  has not been inserted yet in the buckle main body  110  (see FIG.  13  and FIG.  16 ), the holder member  117  is inserted at a left end portion thereof in the opening  132  such that the ridges  151 B, 151 C formed on the curved surface  154  of the holder member  117  is in contact with the face  132 A which defines the left edge of the opening  132 . Incidentally, the portion which defines the left edge of the opening  132  serves as a lock control portion  131  which controls movements of the holder member  117 . 
     The release button  118  is provided with an operating portion  161 , which can be pressed by a user, and arms  162 B, 162 C arranged on opposite sides of the operating portion  161  such that the arms  162 B, 162 C extend out rightward horizontally. The release button  118  is also provided with tilted portions  163 B, 163 C, which serve as drive portions for coming into contact with the abutting portions  159 B, 159 C of the holder member  117  and causing the holder member  117  to the first position. 
     This release button  118  is slidably arranged on the side walls  11 B, 111 C such that the beads  129 B, 129 C formed on the inner walls of the right end portions of the arms  162 B, 162 C are movably supported in the cranked slots  134 B, 134 C formed in the side walls  111 B, 111 C. Namely, this release button  118  is designed such that, when the release button  118  is pressed rightward to release the tongue plate  105  from the buckle main body  110 , the advancement of the release button  118  is facilitated owing to the guiding of the beads  129 B, 129 C by the cranked slots  134 B, 134 C. 
     A raised spring seat  164  for holding the coil spring  119  at a left end portion thereof is formed on the inner end wall of the operating portion  161  at a substantially central part thereof. As the coil spring  119  held by the raised spring seat  164  is fixedly held at a right end portion thereof by the raised spring seat  128  of the spring holder  120 , the release button  118  is biased normally leftward by the coil spring  119 . 
     A description will next be made about a specific operation of the buckle assembly according to the second embodiment. 
     When the tongue plate  105  has not been inserted yet in the buckle main body  110 , the slider  112  is placed on a left side by the biasing force of the coil spring  113  as illustrated in FIG.  13  and FIG.  16 . 
     The holder member  117  is biased in an upper left direction by the coil spring  115 , whereby the support apertures  157 B, 157 C in the holder member  117  and the slots  133 B, 133 C in the side walls  111 B, 111 C are arranged at a certain angle therebetween, respectively. 
     Further, the lock pin  116  is located at the right edges of the support apertures  157 B, 157 C and also at the right edges of the slots  133 B, 133 C. At this time, the latch plate  114  is allowed to retain under the biasing force of the coil spring  115  the state that the latch portion  41  is located in an upper position. 
     To latch the tongue plate  105  in the buckle main body  110 , the tongue plate  105  is next inserted into the buckle main body  110  through the mouth  153 . At this time, the leading end of the tongue plate  105  causes the slider  112  to move rightward so that the coil spring  113  is compressed. When the slider  112  comes into contact at the pressing portions  135 B, 135 C thereof with the legs  144 B, 144 C of the latch plate  114 , the latch plate  114  begins to pivot toward the locking position (in a counterclockwise direction shown in the drawings) about the support portions  146 B, 146 C as fulcrums. 
     Concurrently with the above motion, the holder member  117  begins to turn (move) in response to the pivotal motion of the latch plate  114  while compressing the coil spring  115  with the ridges  151 B, 151 C formed on the curved surface  154  being maintained in contact with the lock control portion  131 . Described specifically, the holder member  117  begins to undergo the above-described turning while being prevented from moving leftward by the lock control member  31 . By this turning of the holder member  117 , the lock pin  116  is pressed by upper edges of the support apertures  157 B, 157 C, so that the lock pin  116  begins to move leftward in the slots  133 B, 133 C of the side walls  111 B, 111 C. Namely, the lock pin  116  undergoes a movement toward the retaining position by the cam action of the support apertures  157 B, 157 C and that of the slots  133 B, 133 C. 
     When the tongue plate  105  is inserted, the latch plate  114  and the holder member  117  turn (move) further. The curved surface  154  of the holder member  117  is then caused to move beyond the lock control portion  131 , whereby the holder member  117  penetrates under the lock control portion  131 . As a result, the holder member  117  becomes movable leftward. As a consequence, the lock pin  116  moves to the left edges of the support apertures  157 B, 157 C of the holder member  117 . As the latching portion  141  of the latch plate  114  has already completed its engagement with the latching aperture  160  of the tongue plate  105  at the time point of the above-described penetration of the holder member  117 , it is possible to prevent the lock pin  116  from being held between the latch plate  114  and the slots  133 B, 133 C of the buckle base  111 . 
     The leftward movement of the holder member  117  continues until the holder member  17  comes under the biasing force of the coil spring  115  into contact with the lock pin  116  moved to the left edges of the slots  133 B, 133 C of the buckle base  111 , and the latching is completed. 
     Incidentally, a force applied toward the unlatching position of the latch plate  114  (in the releasing direction) is borne by the support surface  142  of the latch plate  114 , the lock pin  116 , and the slots  133 B, 133 C. The force is therefore divided, thereby making it possible to further enhance the reliability for strength. 
     Next, upon pulling the tongue plate  105  out of the buckle main body  110 , the release button is pressed rightwards. The tilted portions  163 B, 163 C of the release button  118  then come into contact with the abutting portions  159 B, 159 C of the holder member  117  as shown in FIG.  15 . When the release button  118  is pressed further, the holder member  117  begins to move rightward (in a substantially horizontal direction). The lock pin  116  is pressed via the holder member  117  and begins to move rightward (toward the non-retaining position), whereby the contact between the lock pin  116  and the support surface  142  of the latch plate  114  is canceled. 
     At this time point, the left edge of the upper surface  155  of the holder member  117  is still located on the lower surface of the lock control portion  131 , so that the latch plate  114  is held by the holder member  117 , remains in the latching position and has not moved to the unlatching position. When the release button  118  is pressed still further, the left edge of the upper surface  155  of the holder member  117  is caused to move beyond the lock control portion  131  and disengages from the lower surface of the lock control portion  131 . As a result, it becomes possible for the holder member  117  to move upward in a substantially vertical direction. 
     During these movements, the release button  118  is caused to move with the beads  129 B, 129 C guided by the cranked slots  134 B, 134 C. Since the cranked slots  134 B, 134 C are configured in a substantially crank form as mentioned above, the release button  118  is twisted or prized counterclockwise as viewed in FIG.  15 . 
     Even if a foreign matter such as dirt or sugar-containing juice has penetrated to the inside of the buckle main body  110  and the sliding performance between the tilted portions  163 B, 163 C of the release button  118  and the abutting portions  159 B, 159 C of the holder member  117  has been deteriorated, it is still possible to surely achieve the movement of the holder member  117  toward the first position (in a substantially vertical direction) owing to the above-described twisting (prizing) of the release button  118 . 
     By the above-described action of the release button  118 , the holder member  117 , as is illustrated in FIG. 16, moves at a stretch toward the first position, which corresponds to the unlatching position of the latch plate, by the biasing forces of the coil springs  15 , 13 . 
     At the same time, the lock pin  116  is pressed by the lower edges of the support apertures  157 B, 157 C of the holder member  117  and moves at a stretch to the retaining position (rightward) in the slots  133 B, 133 C of the side walls  111 B, 111 C. Described specifically, the lock pin  116  is caused to move by the cam action of the support apertures  157 B, 157 C and that of the slots  133 B, 133 C. At this time, the lock pin  116  is supported only in the support apertures  157 B, 157 C and the slots  133 B, 133 C and is out of contact with the latch plate  114 . The lock pin  116  therefore moves to a non-retaining position without stopping at an intermediate position. 
     Concurrently, the latch plate  114  pivots toward the unlatching position (in a clockwise direction) by the biasing force of the coil spring  115  about the support portions  146 B, 146 C as fulcrums and moves to the unlatching position, so that the latching of the tongue plate  105  is canceled. 
     In the second embodiment described above, the slots  134 B, 134 C were configured in the substantially crank form. The slots  134 B, 134 C are however not limited to such a crank form, and their shape can be modified, for example, into such a substantially L-shaped form that, as is illustrated in FIG. 18, the slots are bent upward at right parts thereof with respect to substantially central parts thereof as viewed in the horizontal direction or into such a curved form that, as is shown in FIG. 19, the slots are upwardly curved at right parts thereof. In essence, no particular limitation is imposed on the form of the slots  134 B, 134 C insofar as they can induce a twisting or prizing motion of the release button  118  toward the unlatching position. 
     Provided that the release button  118  can be caused to undergo the same motion as that described above, guide ridges or the like may be arranged in place of the cranked slots  134 B, 134 C to guide the beads  129 B, 129 C.