Patent Publication Number: US-11390234-B2

Title: Vehicle seat airbag system and vehicle seat

Description:
The present application is a Divisional Application of U.S. patent application Ser. No. 15/901,067, filed on Feb. 21, 2018, which is a Divisional Application of U.S. patent application Ser. No. 15/181,885, filed on Jun. 14, 2016, now U.S. Pat. No. 9,919,671 B2, issued on Mar. 20, 2018, which is a Divisional Application of U.S. patent application Ser. No. 14/503,665, filed on Oct. 1, 2014, now U.S. Pat. No. 9,409,539 B2, issued on Aug. 9, 2016, which is based on and claims priority from Japanese Patent Application No. 2013-206794, filed on Oct. 1, 2013, Japanese Patent Application No. 2013-206800, filed on Oct. 1, 2013, and Japanese Patent Application No. 2014-163516, filed on Aug. 11, 2014, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a vehicle seat airbag system and a vehicle seat, and particularly to a vehicle seat airbag system and a vehicle seat equipped with an airbag within a seat back of the seat. 
     2. Related Art 
     In a vehicle seat such as an automobile seat, an airbag system is known that protects a seated person against an impact at the time of a collision by inflation and development of an airbag which is disposed in a seat back 
     For example, the vehicle seat disclosed in Japanese Unexamined Patent Application Publication (JP-A) No. 10-273000 includes a seat pad on the front surface of an elastic installation supporting member which is installed inward of a seat back frame, and an airbag in a contracted state is disposed in a space between the elastic installation supporting member and a supporting plate which is disposed rearwardly of the elastic installation supporting member and has a robust structure. 
     With this vehicle seat, an operation of an inflator corresponding to detection of an impact greater than or equal to a predetermined value causes the airbag to inflate between the elastic installation supporting member and the supporting plate so that rearward movement of a seated person at the time of a collision is elastically supported and regulated from the rear, and the impact energy is absorbed by pressure reduction and contraction of the airbag. 
     In Japanese Unexamined Patent Application Publication (JP-A) No. 2010-52621, a first airbag, which is designed to inflate and develop toward a seated person on the rear seat, is installed in the lower portion of the rear surface of the seat back of the front seat, and a second airbag in a flat state is disposed on the front surface of an elastic installation supporting member which is installed inside a seat back frame within the seat back of the front seat. 
     Consequently, an operation of an inflator corresponding to detection of an impact greater than or equal to a predetermined value causes the first airbag to inflate and develop toward a seated person on the rear seat, thus even when the seated person on the rear seat is moved forward due to an impact at the time of a collision, the seated person is prevented from strongly hitting the rear surface of the seat back of the front seat. Furthermore, rearward movement of the upper body of a seated person on the front seat at the time of a collision is elastically supported, regulated from the rear and protected by the inflation and development of the second airbag. The inflation timing for the first and second airbags is controlled, and an appropriate protection for a seated person is achieved. 
     As described above, the vehicle seat disclosed in JP-A No. 10-273000 achieves the protection of a seated person on the vehicle seat by the presence of the air bag and the supporting plate which is disposed outward rearwardly of the air bag and has a robust structure. That is, the impact of rearward movement of a seated person is reduced by the airbag, and an impact applied from the rear of the seat back is received by the supporting plate and is absorbed by the airbag. 
     Therefore, when a rear seat is present and a seated person is on the rear seat, a collision of the seated person with the front seat from the rear of the seat back causes a large impact when the seated person collides with the supporting plate. Therefore, an impact from the rear cannot be received softly, which may cause not only a damage to the knees of the seated person on the rear seat, but also a significant impact to the person on the front seat. 
     On the other hand, according to the airbag device in JP-A No. 2010-52621, protection of the seated person on the front seat and reduction in damage may be expected by the second airbag that inflates and develops within the seat back of the front seat. Also, protection of the seated person on the rear seat is achieved by the first airbag that inflates and develops toward the seated person on the rear seat. However, not only the airbag device needs to be provided within the seat back, but also the first airbag, which inflates and develops toward the seated person on the rear seat, needs to be separately provided in the lower portion of the seat bag of the front seat. Thus, the structure of the seat bag becomes more complicated and control load for each airbag increases. 
     In addition, the first airbag significantly expands diagonally upward from the lower portion of the rear surface of the seat back of the front seat toward the seated person on the rear seat, and thus the inflation and development pattern becomes unstable, which does not provide sufficient stability for accurately, reliably receiving a collision of the seated person on the rear seat with the seat back of the front seat, and for absorbing the impact. 
     As described above, in each structure of the above-described conventional arts, there is a possibility of further improvement on direct protection of the seated person on the front seat by the airbag and protection against an impact to the seat back from the rear by adopting a simple structure. 
     In a vehicle such as an automobile, such a collision of the knees of the seated person on the rear seat with the seat back of the front seat may occur not only at a front-end collision, but also in a situation where the seated person on the rear seat is pushed forward due to deformation of the rear part of the vehicle at the time of a rear-end collision and the knees of the seated person strongly collides with the rear surface of the seat back of the front seat. 
     SUMMARY OF THE INVENTION 
     The present disclosure has been made in view of the above-described problems and provides a vehicle seat airbag system and a vehicle seat that are capable of achieving direct protection of a seated person by an air bag at the time of a collision and accurate reduction in impact from the rear of the seat back without adopting a complicated configuration. 
     A first aspect of the disclosure provides a vehicle seat airbag system including: an airbag which is caused by the seat airbag system to inflate and develop within a seat back. The airbag is disposed between a seat pad and an outer layer of a rear of the seat back, and the inflation and development of the airbag in the seat back allows the airbag to elastically support an upper body of a seated person and to elastically receive an impact force which is applied to the rear of the seat back. 
     The seat back may have a flexible member that covers a front of the inflated and developed airbag to allow the upper body of a seated person to be elastically supported, and the seat back may further have a flexible movable member that covers a back of the inflated and developed airbag to allow an impact force applied to the rear of the seat back to be elastically received. 
     A second aspect of the disclosure provides a vehicle seat airbag system including an airbag which is caused by the seat airbag system to inflate and develop within a seat back. The airbag is disposed between a seat pad and an outer layer of a rear of the seat back, and the inflation and development of the airbag in the seat back causes the outer layer to expand rearwardly and allows the airbag to elastically support an upper body of a seated person and to elastically receive an impact force which is applied to the rear of the seat back. 
     The outer layer may have an elastic portion in an area corresponding to the inflated and developed airbag. 
     At least an area of the outer layer may be composed of a material having elasticity different from elasticity of the outer layer other than the area which corresponds to the inflated and developed airbag. 
     The outer layer may have a gusset that enables rearward expansion of the airbag which is caused by the inflation and development of the airbag. The outer layer may have a breaking part that enables rearward expansion of the airbag which is caused by the inflation and development of the airbag. 
     The vehicle seat airbag system may further include a rear board that is disposed rearwardly of the outer layer along an expansion area of the outer layer. 
     A third aspect of the disclosure provides a vehicle seat airbag system including an airbag which is caused by a seat airbag system to inflate and develop within a seat back. The airbag is disposed between a seat pad and a rear board provided in a rear of the seat back, and the inflation and development of the airbag in the seat back allows the airbag to elastically support an upper body of a seated person, and causes the rear board to be pushed rearwardly to allow the airbag to elastically receive an impact force which is applied to the rear board. 
     The rear board may be supported on the rear of the seat back swingably in a direction away from the rear of the seat back. 
     The rear board may be removed from the rear of the seat back by the pushing. 
     The rear board may be removably fixed to the rear of the seat back and include: a collision prediction unit that predicts a collision to the vehicle; and a fixing release mechanism that releases the fixing based on information of collision occurrence which is predicted by the collision prediction unit. 
     An outer layer may be interposed between the airbag and the rear board, the outer layer being caused by the inflated and developed airbag to expand rearwardly, and the outer layer may be provided with the rear board. 
     The rear board may include a fragile part. 
     The vehicle seat airbag system may further include: a control unit that controls the inflation and development of the airbag; and a collision detection unit that detects a collision to the vehicle. The control unit may cause the airbag to inflate and develop after a predetermined time delay since the collision detection unit detects a collision. 
     The vehicle seat airbag system may further include: a control unit that controls the inflation and development of the airbag; and a collision prediction unit that predicts a collision to the vehicle. The control unit may start to cause the airbag to inflate and develop at a time which is determined based on a time of collision occurrence predicted by the collision prediction unit. 
     The airbag may inflate and develop in multiple steps within the seat back. 
     An fourth aspect of the disclosure provides a vehicle seat including: a seat back frame, an elastic installation supporting member installed in the seat back frame, and an airbag within a seat back, the vehicle seat causing the airbag to inflate and develop within the seat back. An inflator is attached to the seat back frame, and the inflation and development of the airbag in the seat back due to an operation of the inflator allows the airbag to elastically support an upper body of a seated person and to elastically receive an impact force which is applied to a rear of the seat back. 
     The seat back frame may be formed in a substantially frame shape along a seat back shape, the airbag is provided in the seat back frame in a substantially frame shape, and the inflator may be attached to a side portion of the seat back frame in a substantially frame shape. 
     The inflator may be disposed within the seat back frame in a substantially frame shape. 
     The airbag may be disposed in the vehicle body rearwardly of the elastic installation supporting member, and the inflator may be disposed in the vehicle body forwardly of the elastic installation supporting member. 
     The inflator may be disposed outside the seat back frame in a substantially frame shape. 
     A fifth aspect of the disclosure provides a vehicle seat including: a seat back frame, an elastic installation supporting member installed in the seat back frame, and an airbag within a seat back, the vehicle seat causing the airbag to inflate and develop within the seat back. An inflator is attached to the seat back frame, the seat back frame is formed in a substantially frame shape along a seat back shape, the airbag is provided in the seat back frame in a substantially frame shape, the inflator is attached to at least one of an upper portion and a lower portion of the seat back frame in a substantially frame shape, and the inflation and development of the airbag in the seat back due to an operation of the inflator allows the airbag to elastically support an upper body of a seated person and to elastically receive an impact force which is applied to a rear of the seat back. 
     The inflator may be disposed within the seat back frame. 
     The inflator may be disposed outside the seat back frame. 
     The inflator may be attached to a position near a side end of at least one of the upper portion and the lower portion of the seat back frame in a substantially frame shape. 
     The inflator may be attached to each of the upper portion and the lower portion of the seat back frame, and each inflator may operate with a time lag to cause the inflation and development. 
     The lower portion of the seat back frame to which the inflator is attached may be a cylindrical cross member that extends in a width direction along a lower side of the seat back, and the inflator may be fixed to an inside of the cross member. 
     A sixth aspect of the disclosure provides a vehicle seat including: a seat back frame, an elastic installation supporting member installed in the seat back frame, and an airbag within a seat back, the vehicle seat causing the airbag to inflate and develop within the seat back. An inflator is attached to the seat back frame, the seat back frame is formed as a whole in a substantially frame shape along a seat back shape, the airbag is provided in the seat back frame in a substantially frame shape, the inflator is attached to the seat back frame in a substantially frame shape at a corner portion or a bending portion thereof, and the inflation and development of the airbag in the seat back due to an operation of the inflator allows the airbag to elastically support an upper body of a seated person and to elastically receive an impact force which is applied to a rear of the seat back. 
     The attachment of the inflator to the seat back frame may be achieved by bridging the inflator between both side positions of the seat back frame including the corner portion or the bending portion. 
     The seat back frame may have a lower cross member and an upper cross member that extend in a width direction at a lower and an upper positions respectively, and a pair of side frames that are disposed on both sides in the width direction. The corner portion or the bending portion may serve as a connecting portion between the side frames and the lower cross member. The inflator may be bridged between and attached to the side frames and the lower cross member. 
     The seat back frame may have a lower cross member and an upper cross member that extend in a width direction at a lower and an upper positions respectively, and a pair of side frames that are disposed on both sides in the width direction. The corner portion or the bending portion may be located between the side frames and the upper frame. The inflator may be bridged between and attached to the side frames and the upper cross member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic vertical cross-sectional view of a vehicle seat illustrating the outline of a vehicle seat airbag system according to a first implementation; 
         FIG. 2  is a schematic perspective view with partial cutaway of the vehicle seat as seen from the rear; 
         FIG. 3  is an operation explanatory diagram of the vehicle seat when an impact occurs; 
         FIG. 4  is a control circuit block diagram of the vehicle seat airbag system; 
         FIG. 5  is a control circuit block diagram of the vehicle seat airbag system; 
         FIG. 6  is a schematic explanatory diagram as seen from the rear, illustrating the outline of a second implementation; 
         FIG. 7  is an operation explanatory diagram of the vehicle seat when an impact occurs; 
         FIG. 8  is an operation explanatory diagram of the vehicle seat; 
         FIG. 9A  is a schematic explanatory diagram of the rear part of a seat back and  FIG. 9B  is an operation explanatory diagram of the seat back; 
         FIG. 10A  is a schematic explanatory diagram of the rear part of the seat back and  FIG. 10B  is an operation explanatory diagram of the seat back; 
         FIG. 11A  is a schematic explanatory diagram of the rear part of the seat back and  FIG. 11B  is an operation explanatory diagram of the seat back; 
         FIG. 12A  is a schematic explanatory diagram of the rear part of the seat back and  FIG. 12B  is an operation explanatory diagram of the seat back; 
         FIG. 13A  is a schematic explanatory diagram of the rear part of the seat back and  FIG. 13B  is an operation explanatory diagram of the seat back; 
         FIG. 14A  is a schematic explanatory diagram of the rear part of the seat back and  FIG. 14B  is an operation explanatory diagram of the seat back; 
         FIG. 15A  is a schematic explanatory diagram of the rear part of the seat back and  FIG. 15B  is an operation explanatory diagram of the seat back; 
         FIG. 16A  is a schematic explanatory diagram of the rear part of the seat back and  FIG. 16B  is an operation explanatory diagram of the seat back; 
         FIG. 17A  is a schematic explanatory diagram of the rear part of the seat back and  FIG. 17B  is an operation explanatory diagram of the seat back; 
         FIG. 18  is a schematic vertical cross-sectional view illustrating the outline of a vehicle seat according to a third implementation; 
         FIG. 19  is a schematic perspective view with partial cutaway of the vehicle seat as seen from the rear; 
         FIG. 20A  is an enlarged view of portion a of  FIG. 18  illustrating the outline of a rear board supporter and  FIG. 20B  is an enlarged view of portion b of  FIG. 18  illustrating the outline of a rear board retainer; 
         FIG. 21  is an operation explanatory diagram of the vehicle seat when an impact occurs; 
         FIG. 22  is a schematic explanatory diagram illustrating another example of the third implementation; 
         FIG. 23  is a schematic vertical cross-sectional view illustrating the outline of a vehicle seat according to a fourth implementation; 
         FIG. 24A  is an enlarged perspective view of portion c of  FIG. 23  illustrating the outline of a rear board supporter and  FIG. 24B  is an enlarged view of portion d of  FIG. 23  illustrating the outline of a rear board retainer; 
         FIG. 25  is an operation explanatory diagram of a vehicle seat when an impact occurs; 
         FIG. 26  is a schematic vertical cross-sectional view illustrating the outline of a vehicle seat according to a fifth implementation; 
         FIG. 27  is an operation explanatory diagram of a vehicle seat when an impact occurs; 
         FIG. 28  is a schematic rear view with partial cutaway for explaining a sixth implementation; 
         FIG. 29  is a cross-sectional view of  FIG. 28  taken along line XXIX-XXIX; 
         FIG. 30  is a schematic cross-sectional view of a vehicle seat according to a seventh implementation; 
         FIG. 31  is a schematic perspective view with partial cutaway of the vehicle seat as seen from the rear; 
         FIG. 32  is an operation explanatory diagram of the vehicle seat when an impact occurs; 
         FIG. 33  is a schematic perspective view with partial cutaway of a vehicle seat as seen from the rear according to an eighth implementation; 
         FIG. 34  is a fragmentary perspective view with partial cutaway of the vehicle seat; 
         FIG. 35  is a schematic rear view with partial cutaway for explaining a modification of the eighth implementation; 
         FIG. 36  is an operation explanatory diagram of the vehicle seat when an impact occurs according to a modification of the eighth implementation; 
         FIG. 37  is a schematic cross-sectional view of a vehicle seat according to a ninth implementation; 
         FIG. 38  is a schematic perspective view with partial cutaway of the vehicle seat as seen from the rear; 
         FIG. 39  is an operation explanatory diagram of the vehicle seat when an impact occurs; 
         FIG. 40  is a schematic perspective view with partial cutaway of a vehicle seat as seen from the rear according to a tenth implementation; and 
         FIG. 41  is a schematic cross-sectional view of an operation state of the vehicle seat at the time of an offset collision, the cross-sectional view corresponding to a cross-section along line XXXXI-XXXXI in  FIG. 40 . 
     
    
    
     DETAILED DESCRIPTION 
     First Implementation 
     Hereinafter, a first implementation of a vehicle seat airbag system according to the present disclosure will be described with reference to the accompanying drawings.  FIG. 1  is a schematic vertical cross-sectional view illustrating the outline of a vehicle seat;  FIG. 2  is a schematic perspective view with partial cutaway of the seat for a vehicle as seen from the rear; and  FIG. 3  is an operation explanatory diagram of the seat for a vehicle when an impact occurs. It is to be noted that arrow F indicates the forward direction of the vehicle and arrow W indicates a vehicle width direction in each figure. 
     As illustrated in  FIG. 1 , a front seat  10  and a rear seat  60  are disposed in parallel at the front and rear as vehicle seats on a floor  1  in the vehicle cabin. The front seat  10  has a seat cushion  11  that is supported on the floor  1  and supports the buttocks of a seated person Pf, a seat back  20  that supports an upper body Pb which is from a waist to a chest of the seated person Pf, and a headrest  40  which is located upwardly of the seat back  20  and supported on the seat back  20 . Although the seated person Pf may rest a head Ph on the headrest  40  while driving a vehicle, this posture makes it difficult to perform driving operations, and so normally, the seated person Pf is seated with the head Ph slightly away from the headrest  40 . 
     As illustrated in  FIGS. 1 and 2 , in the front seat  10 , a plurality of elastic installation supporting members  27  is provided in a seat back frame  21  that forms the framework of the seat back  20 , a seat pad  28  is disposed on the front surface of the elastic installation supporting members  27 , an airbag  30  in a contracted state is disposed rearwardly of the elastic installation supporting members  27 , and all of these components are covered by an outer layer  31  in a bag shape. 
     The seat back frame  21  is formed in a substantially rectangular frame shape by a U-shaped tube frame, a pair of right and left side brackets  24 , a tube-shaped upper cross member  25 , and a lower cross member  26 , the U-shaped tube frame including an upper frame  22  extending in a vehicle width direction and a pair of side frames  23  curving or bending downward from both ends of the upper frame  22  and extending downward, the pair of right and left side brackets  24  being fixedly disposed at the side edges of the side frames  23 , the tube-shaped upper cross member  25  extending in a vehicle width direction and being installed between the vicinities of the upper ends of the right and left side frames  23 , the lower cross member  26  being installed between the vicinities of the lower ends of the right and left side brackets  24 . 
     A plurality of the elastic installation supporting members  27  such as zigzag springs made of metal is provided to be bridged between the opposed side frames  23  and between the side brackets  24 . In the present implementation, three elastic installation supporting members  27  are provided to be bridged therebetween at substantially the same intervals. 
     The seat pad  28 , which elastically supports the upper body Pb of the seated person Pf, is disposed on the front surface of the elastic installation supporting members  27 , the seat pad being composed of a urethane foam material. The airbag  30 , which is flatly folded in a rectangular shape in a contracted state, is disposed in the rear of the elastic installation supporting members  27 . 
     The whole thing including the seat back frame  21 , the seat pad  28 , and the air bag  30  is covered by the outer layer  31  in a bag shape having a front surface  32  that covers the front of the seat pad  28  and a rear part  33  that covers the air bag  30  with an expandable/contractible and flexible fabric (textile, knit, non-woven fabric) or leather (natural leather, synthetic leather), thereby forming the seat back  20 . A rear board, which is a movable member, may be disposed at the rear portion. The air bag  30  is held in, for example, the outer layer  31  by a retaining unit (not illustrated) so as to allow inflation and development. The air bag  30  is provided with a gas inlet (not illustrated) and is connected to a cylindrical inflator  29  which is attached to a side surface  24   a  outward of the side bracket  24  in the vehicle body. 
     The base end of each side bracket  24  included in the seat back frame  21  of the seat back  20  is attached to a rotation arm  16  of a reclining device  15  disposed in the rear of the seat cushion  11  by screwing a mounting bolt  17 , and thus the seat back frame  21  is integrally connected to the rotation arm  16  of the reclining device  15 . It is to be noted that existing publicly known reclining device  15  may be used, which is not directly related to the present disclosure and thus detailed description is omitted. 
     As illustrated in  FIGS. 1 and 2 , a pair of right and left cylindrical stay brackets  42  is integrally formed with the upper frame  22  of the seat back frame  21 . Stays  41  mounted in the headrest  40  are inserted in and retained to the stay brackets  42  via a headrest holder, and thus the headrest  40  is mounted on the upper end of the seat back  20 . 
     Thus, as illustrated in  FIGS. 1 and 2 , the airbag  30  contracted in a substantially rectangular plane shape is disposed between the elastic installation supporting members  27  and the rear part  33  of the outer layer  31  and thus is incorporated in the seat back  20 . The seat back  20  covers the front of the expandable and developable airbag  30  with the elastic installation supporting members  27  capable of elastically supporting rearward movement of the seated person Pf, and flexible members such as the seat pad  28  and the front surface  32  of the outer layer  31 . On the other hand, the seat back  20  covers the rear of the airbag  30  with the rear part  33  of the outer layer  31  which a flexible member or a movable member capable of elastically receiving an impact force which is applied to the rear of the seat back. 
     The airbag  30  is designed to instantly expand and develop two-dimensionally due to expanded gas injection of the inflator  29 . The airbag  30  has orifices (not illustrated) that release internal gas to the outside. The diameter and number of orifices are set so as to allow the internal gas to be released gradually with the contraction of the airbag  30  after the inflation and development thereof. 
     This configuration ensures bending deformation of the elastic installation supporting members  27  and the seat pad  28  which are provided within the seat back frame  21  disposed within the seat back  20 , and thus elastic support of the seated person Pf in a normal seating state is appropriately obtained and favorable seating performance may be ensured. 
     On the other hand, when the airbag  30  inflates and develops two-dimensionally between the elastic installation supporting members  27  and the rear part  33  of the outer layer  31  due to expanded gas injection of the inflator  29 , as illustrated in  FIG. 3 , the airbag  30  pushes forward and urges the elastic installation supporting members  27  and the seat pad  28  so as to allow the upper body Pb of the seated person Pf to be elastically supported from the rear side, and the airbag  30  further develops and causes the rear part  33  of the outer layer  31  to expand rearwardly so as to allow an impact force applied to the rear part of the seat back  20  to be elastically received. The inflation and development of the airbag  30  is covered by the elastic installation supporting members  27  that hold the airbag  30  and the rear part  33  of the outer layer  31  that extends and expands, and thus development behavior and development pattern are controlled and stable inflation and development are maintained, and also sufficient volume of inflation and development is obtained, and the energy absorbing stroke of the airbag  30  in a fore-and-aft direction of the vehicle body may be ensured. 
       FIG. 4  illustrates a control circuit of the vehicle seat airbag system. The control circuit includes a control unit  50  that controls inflation and development of the airbag  30 , and the control unit  50  is electrically connected to a collision detection sensor  51  and the inflator  29  to control the operation of the inflator  29  according to collision occurrence information from the collision detection sensor  51  and a program stored in a ROM  53 , the collision detection sensor being a collision detection unit to detect a collision to the rear of a vehicle, that is, an impact of a rear-end collision. 
     The collision detection sensor  51  includes an acceleration sensor that issues a signal according to acceleration which represents an impact force at the time of a rear-end collision. The control unit  50  determines whether or not a rear-end collision has occurred based on comparison between acceleration detected by the collision detection sensor  51  and a predetermined threshold value. Based on the determination of rear-end collision occurrence, the control portion  50  operates the inflator  29  to cause the airbag  30  to expand and develop when a rear-end collision has occurred. 
     The operation of the vehicle airbag system having the above configuration will be described. 
     In a vehicle equipped with the vehicle airbag system, when an impact greater than or equal to a predetermined impact is applied to the vehicle due to a rear-end collision or the like, the impact is detected by the collision detection sensor  51 , and the control unit  50  outputs a drive signal to the inflator  29 , which is ignited and expanded gas jets from the inflator  29 . In this manner, as illustrated in  FIG. 3 , the airbag  30  instantly inflates and develops between the elastic installation supporting members  27  and the rear part  33  of the outer layer  31 . Thus, rearward movement, due to a rear-end collision, of the upper body Pb of the seated person Pf on the front seat  10  is elastically received and regulated by the airbag  30  via the front surface  32  of the outer layer  31 , the seat pad  28 , and the elastic installation supporting members  27 . 
     On the other hand, because of vehicle body deformation due to an impact and/or inertia due to a rear-end collision, the upper body of the seated person Pr on the rear seat  60  is moved rearward and pressed against the seat back, then is moved linearly forward by rebounding. Then, for example, knees Pn come into contact with the rear part of the seat back  20  of the front seat  10 , and an impact force is applied to the rear part of the front seat  10 . At this point, the knees Pn are elastically received by the airbag  30  which has expanded and developed between the elastic installation supporting members  27  and the rear part  33  of the outer layer  31  in the seat back  20 , and thus forward movement are regulated, and the upper body Pb of the seated person Pf on the front seat  10  avoids receiving an impact force from the knees Pn of the seated person Pr on the rear seat  60 , and consequently the seated person Pf on the front seat  10  is protected. Similarly, the knees Pn of the seated person Pr moving forward on the rear seat  60  are elastically received and protected by the airbag  30  which inflates and develops. 
     In the above description, when the collision detection sensor  51  detects an impact greater than or equal to a predetermined impact, the control unit  50  instantly operates and controls the inflator  29  to cause the airbag  30  to expand and develop. The control unit  50 , however, may control the airbag  30  so that the airbag  30  inflates and develops after a predetermined time delay since the collision detection unit  51  detects an impact. That is, the control unit  50  determines whether or not a rear-end collision has occurred based on comparison between acceleration detected by the collision detection sensor  51  and a predetermined threshold value, and operates the inflator  29  to cause the airbag  30  to expand and develop after elapse of a predetermined time since a rear-end collision occurred. 
     In this manner, the airbag  30  inflates and develops after elapse of a predetermined time since an occurrence of a rear-end collision, and so at an occurrence of a rear-end collision, the seat pad  28  of the seat back  20  and the elastic installation supporting members  27  undergo bending deformation because of inertia, and the upper body Pb of the seated person Pf on the front seat  10  is moved rearward quickly to be pressed into the seat back  20 . Then the head Ph is instantly restrained and supported by the headrest  40  after the collision and thus a load applied to the neck is reduced. Subsequently, due to expanded gas injection of the inflator  29 , the airbag  30  inflates and develops two-dimensionally between the elastic installation supporting members  27  and the outer layer  31  that forms the rear part of the seat back  20 , then the airbag  30  pushes forward and urges the elastic installation supporting members  27  and the seat pad  28  so as to elastically support the upper body Pb of the seated person Pf, then develops and causes the rear part  33  of the outer layer  31  to expand rearwardly so as to allow an impact force applied to the rear part of the seat back  20  to be elastically received. Accordingly, rearward movement, due to a rear-end collision, of the upper body Pb of the seated person Pf on the front seat  10  is elastically received and rearward movement is regulated by the airbag  30  via the outer layer  31  of the seat back  20 , the seat pad  28 , and the elastic installation supporting members  27 , the airbag  30  ensuring the amount of development and inflation, and the energy absorbing stroke in a fore-and-aft direction of the vehicle body. 
     In this manner, similarly to what has been described above, the seated person Pf on the front seat  10  and the seated person Pr on the rear seat  60  are protected by the airbag  30  which inflates and develops, and at an occurrence of a rear-end collision, the upper body Pb of the seated person Pf on the front seat  10  is moved rearward quickly to be pressed into the seat back  20 , then the head Ph is supported by the headrest  40  to reduce a load applied to the neck, and thus occurrence of whiplash injury of the seated person Pf is reduced. 
     This vehicle seat airbag system may control its operation using a collision prediction unit instead of the above-mentioned control circuit. 
       FIG. 5  illustrates the control circuit of the vehicle seat airbag system. The control circuit includes a control unit  55  that controls inflation and development of the airbag  30 , and the control unit  55  controls the operation of the inflator  29  according to collision occurrence information from the collision detection sensor  56  and a program stored in a ROM  57 , the collision detection sensor being a collision prediction unit to predict a rear-end collision and an impact load to a vehicle. 
     Here, the control unit  55  has a built-in timer and measures time by starting the timer from a rear-end collision predicted time. Also, the collision detection sensor  56  includes, for example, a distance sensor such as a millimeter wave sensor, and predicts a rear-end collision by measuring a relative distance and/or a relative speed between the self-vehicle and another vehicle with which a rear-end collision may occur. In addition, the collision detection sensor  56  predicts an impact load at the occurrence of predicted rear-end collision. 
     In this manner, at a predicted occurrence time of collision, similarly to what has been described above, as illustrated in  FIG. 3 , the airbag  30  instantly inflates and develops between the elastic installation supporting members  27  and the rear part  33  of the outer layer  31 , and rearward movement of the upper body Pb of the seated person Pf on the front seat  10  is elastically received and the rearward movement is regulated. On the other hand, even when the knees Pn of the seated person Pr on the rear seat  60  come into contact with the rear part of the seat back  20  of the front seat  10 , the knees Pn are elastically received by the airbag  30  which has expanded and developed and rearward movement is regulated, and thus the seated person Pf on the front seat  10  is protected. 
     In addition, when inevitability of a rear-end collision of a vehicle is predicted based on signals from the collision detection sensor  56 , the control unit  55  may control the airbag  30  so that the airbag  30  starts to expand and develop after a predetermined time delay since the predicted occurrence time of collision. That is, when the control unit  55  predicts inevitability of a rear-end collision of the vehicle based on signals from the collision detection sensor  56 , inflation and development of the airbag  30  starts after elapse of a predetermined time since the predicted occurrence time of collision. 
     In this manner, when a rear-end collision occurs at a predicted occurrence time of collision, the seat pad  28  of the seat back  20  and the elastic installation supporting members  27  undergo bending deformation because of the inertia of the seated person Pf, and the upper body Pb of the seated person Pf on the front seat  10  is moved rearward quickly to be pressed into the seat back  20 , then the head Ph is supported by the headrest  40  instantly after the collision. The airbag  30  inflates and develops after elapse of a predetermined time since a subsequent predicted occurrence time of collision. Thus, rearward movement, due to a rear-end collision, of the upper body Pb of the seated person Pf on the front seat  10  is elastically received by the airbag  30  via the outer layer  31  of the seat back  20 , the seat pad  28 , and the elastic installation supporting members  27 , and so the rearward movement is regulated. In this manner, similarly to what has been described above, the seated person Pf on the front seat  10  and the seated person Pr on the rear seat  60  are protected by the airbag  30  which inflates and develops, and at an occurrence of a rear-end collision, the upper body Pb of the seated person Pf on the front seat  10  is moved rearward quickly to be pressed into the seat back  20 , then the head Ph is supported by the headrest  40  to reduce a load applied to the neck, and thus occurrence of whiplash injury of the seated person Pf is reduced. 
     Furthermore, when inevitability of a rear-end collision of a vehicle is predicted based on signals from the collision detection sensor  56 , the control unit  55  may control the airbag  30  so that the airbag  30  starts to expand and develop a predetermined time delay before the predicted occurrence time of collision. That is, when the control unit  55  predicts inevitability of a rear-end collision of the vehicle based on signals from the collision detection sensor  56 , inflation and development of the airbag  30  starts a predetermined time before the predicted occurrence time of collision. 
     According to this, a predetermined time before a predicted occurrence time of collision, that is before a collision occurrence, the airbag  30  starts to expand and develop between the elastic installation supporting members  27  and the rear part  33  of the outer layer  31 , the upper body Pb of the seated person Pf is pushed forward via the seat pad  28  and the front surface  32  of the outer layer  31  by the airbag  30  which inflates and develops, thereby causing the back of the seated person Pf to be stretched and the seated position and the seated posture of the seated person Pf is made to be suitable. 
     The airbag  30  further inflates and develops with the suitable seated position and seated posture of the seated person Pf, and thus optimal effect of inflation and development of the airbag  30  for the seated person Pf is obtained, and rearward movement of the upper body Pb of the seated person Pf on the front seat  10  at an occurrence of a rear-end collision is elastically received by the airbag  30  and the rearward movement is regulated. 
     Consequently, according to the present implementation, the airbag  30  disposed within the seat back  20  inflates and develops within the seat back  20 , thereby making it possible to elastically receive rearward movement of the seated person Pf due to a collision and to protect the seated person pf on the vehicle seat against an impact from the rear. Thus, without adopting a complicated configuration, direct protection of a seated person by the airbag at the time of a collision and accurate reduction of an impact from the rear of the seat back may be achieved. 
     Moreover, because the inflator  29  is attached to the side surface  24   a  outward of the side bracket  24  of the seat back frame  21  in the vehicle body, the seat back frame being a frame member disposed so as to ensure seating performance, and thus uncomfortable feeling for a seated person caused by provision of the inflator  29  is completely insensible, and the support rigidity of the inflator  29  is ensured. 
     Furthermore, the inflator  29  is disposed outside the seat back frame  21 , and thus interference between the inflator  29  and the airbag  30  which inflates and develops within the seat back  20  is completely avoided, and thus stable development of the airbag  30  is ensured. 
     Also, when the front seat  10  is provided with a side air bag, the inflator  29  may be used in common as an inflator of the side air bag, and the efficiency of the entire air bag device provided in the front seat  10  may be increased. 
     When a lumbar support mechanism is provided within the seat back frame  21  of the front seat  10 , the operation of the lumbar support mechanism is not effected. 
     According to this implementation, the inflator  29  is not located between the opposed side brackets  24  or between the opposed side frames  23 , thereby providing the effect that attachment of the elastic installation supporting members  27  to be installed between the side brackets  24  or the side frames  23  is not interfered. 
     As described above, the operation of the airbag  30  at an occurrence of collision to the front seat  10  has been described by taking a rear-end collision of a vehicle as an example. However, a collision is not limited to a rear-end collision. For example, even in the case of a front-end collision, although operational steps are reversed, similarly, the inflation and development of the airbag  30  first allows the front seat  10  to elastically receive an impact due to forward movement of the knees Pn of the seated person Pr on the rear seat, and subsequently, the front seat  10  is able to elastically receive rearward movement of the upper body Pb of the seated person Pf on the front seat  10 . 
     Second Implementation 
     A second implementation will be described with reference to  FIGS. 6 to 17 .  FIG. 6  is a schematic explanatory diagram as seen from the rear, illustrating the outline of the second implementation;  FIG. 7  is an operation explanatory diagram of the vehicle seat when an impact occurs; and  FIG. 8  is an operation explanatory diagram of the vehicle seat. It is to be noted that the components corresponding to those in the first implementation are denoted by the same symbols in  FIGS. 1 to 5 , and detailed description of the components is omitted. 
     Similarly to the first implementation, in a front seat  10 , a plurality of elastic installation supporting members  27  is provided in a seat back frame  21  that forms a framework of a seat back  20 , a seat pad  28  is disposed on the front surface of the elastic installation supporting members  27 , an airbag  30  in a contracted state is disposed rearwardly of the elastic installation supporting members  27 , and all of these components are covered by an outer layer  31  in a bag shape. 
     As illustrated in  FIG. 6 , in a rear part  33  of the outer layer  31 , a substantially rectangular airbag corresponding area  74 A, which corresponds to the airbag  30 , is composed of an outer layer material which is superior in elasticity compared with the outer layer material of a rear part area  74 B which surrounds the airbag corresponding area  74 A. Thus, an elastic portion is formed correspondingly to the airbag  30 . 
     On the other hand, when the airbag  30  inflates and develops two-dimensionally between the elastic installation supporting members  27  and the rear part  33  of the outer layer  31  due to expanded gas injection of the inflator  29 , as illustrated in the cross-sectional view of  FIG. 7  and in the main perspective view of the rear part  33  of  FIG. 8 , the airbag  30  pushes forward and urges the elastic installation supporting members  27  and the seat pad  28  so as to allow the upper body Pb of the seated person Pf to be elastically supported from the rear side. On the other hand, in the rear part  33  of the outer layer  31 , particularly the area  74 A corresponding to the airbag  30  expands rearwardly in a rectangular dome-shaped, the volume of inflation and development of the airbag  30  is sufficiently ensured, and the airbag  30  develops to allow an impact force applied to the rear part of the seat back  20  to be elastically received, the airbag being composed of an outer layer material superior in elasticity. 
     The inflation and development of the airbag  30  is covered by the elastic installation supporting members  27  that hold the airbag  30  and the rear part  33  of the outer layer  31  that extends and expands, and thus development behavior and development pattern are controlled and stable inflation and development are maintained. In addition, the expandable/contractible elastic installation supporting members  27  and the rear part  33  of the outer layer  31  are caused to expand, inflate and develop, thereby making it possible to obtain the volume of inflation and development of the airbag  30 , and the energy absorbing stroke in a fore-and-aft direction of the vehicle body in the airbag  30  may be ensured. 
     In a vehicle provided with vehicle seats having the above configuration, when an impact greater than or equal to a predetermined impact is applied to the vehicle due to a rear-end collision or the like, the impact is detected by the collision detection sensor, and the control unit outputs a drive signal to the inflator  29 , which is ignited and expanded gas jets from the inflator  29 . In this manner, as illustrated in  FIGS. 7 and 8 , the airbag  30  instantly inflates and develops between the elastic installation supporting members  27  and the rear part  33  of the outer layer  31 . Thus, rearward movement, due to a rear-end collision, of the upper body Pb of the seated person Pf on the front seat  10  is elastically received by the airbag  30  via the front surface  32  of the outer layer  31 , the seat pad  28 , and the elastic installation supporting members  27 , and so the rearward movement is regulated. 
     On the other hand, the seated person Pr on the rear seat  60  may be moved rearward due to a rear-end collision and pressed against the seat back, then is moved linearly forward by rebounding. Then, for example, the knees Pn may come into contact with the rear part of the seat back  20  of the front seat  10 . At this point, the knees Pn are elastically received via the outer layer  31  which inflates and develops by the airbag  30  which has expanded and developed between the elastic installation supporting members  27  and the rear part  33  of the outer layer  31  in the seat back  20 , and thus forward movement is restrained and the upper body Pb of the seated person Pf on the front seat  10  is protected against an impact force F 1  from the knees Pn of the seated person Pr on the rear seat  60 . Similarly, the impact force F 1  from the knees Pn and an impact force F 2  from the upper body of the seated person Pr moving forward on the rear seat  60  are elastically received and protected by the airbag  30  which has expanded and developed and ensures the volume of inflation and development and the energy absorbing stroke. 
     In the rear part  33  which is of the outer layer  31  of the seat back  20  and expands along with the inflation and development of the airbag  30 , as illustrated in  FIG. 9A , a rear part area  81 B, which surrounds a substantially rectangular airbag corresponding area  81 A, may be composed of an outer layer material which is superior in elasticity compared with the outer layer material of the airbag corresponding area  81 A corresponding to the airbag  30 . Thus, an elastic portion may be formed in the rear part area  81 B surrounding the airbag corresponding area  81 A. 
     In this manner, the airbag  30  inflates and develops between the elastic installation supporting members  27  and the rear part  33  of the outer layer  31  due to a rear-end collision or the like. As illustrated in  FIG. 9B , the airbag  30  inflates and develops so as to expand due to extension of the rear part area  81 B which surrounds the airbag corresponding area  81 A included in the rear part  33 . Thus, rearward movement, due to a rear-end collision, of the upper body Pb of the seated person Pf on the front seat  10  is elastically received by the airbag  30  and the rearward movement is regulated. 
     On the other hand, the knees Pn of the seated person Pr on the rear seat  60  is elastically received by the airbag  30  which has expanded and developed between the elastic installation supporting members  27  and the outer layer  31 , and thus the upper body Pb of the seated person Pf on the front seat  10  is protected against the impact force F 1 . 
     Similarly, the impact force F 1  from the knees Pn and the impact force F 2  from the upper body of the seated person Pr moving forward on the rear seat  60  are elastically received and protected by the airbag  30  which has expanded and developed. 
     The inflation and development of the airbag  30  are controlled by the elastic installation supporting members  27  and mainly the rear part  33  of the outer layer  31 , that expands due to extension of the rear part area  81 B surrounding the airbag corresponding area  81 A, and thus stable inflation and development are maintained. 
     Another rear part  33 , which is of the outer layer  31  of the seat back  20  and expands along with the inflation and development of the airbag  30 , has an airbag corresponding area  82 A corresponding to the airbag  30  of the rear part  33  and a rear part area  82 B surrounding the airbag corresponding area  82 A as illustrated in  FIG. 10A , the airbag corresponding area  82 A being defined as a substantially U-shape along both side edges  82   b  and a lower edge  82   c , the rear part area  82 B having both side edges  82   e  and a lower edge  82   f , the both side edges  82   e  being gradually away from the both side edges  82   b  as being located downward from the upper end of each side edge  82   b , the lower edge  82   f  extending from the lower edge  82   c  with a predetermined distance apart and having both ends connected with the lower ends of the both side edges  82   e . The airbag corresponding area  82 A is formed in a lid shape having an upper edge  82   a  which is connected with the rear part area  82 B. 
     A pair of elastic side areas  82 C and a strip-shaped lower elastic area  82 D are integrally formed, the elastic side areas  82 C being formed by placing a substantially upwardly tapered outer layer material superior in elasticity between the both side edges  82   b  of the airbag corresponding area  82 A and the both side edges  82   e  of the rear part area  82 B and by seaming the outer layer material and corresponding both side edges together, the lower elastic area  82 D being formed by placing the outer layer material between the lower edge  82   c  of the airbag corresponding area  82 A and the lower edge  82   f  of the rear part area  82 B and by seaming the outer layer material and corresponding lower edges together. 
     In this manner, the airbag  30  inflates and develops between the elastic installation supporting members  27  and the rear part  33  of the outer layer  31  due to a rear-end collision or the like. As illustrated in  FIG. 10B , when the airbag corresponding area  82 A included in the rear part  33  is pressed, the elastic both side areas  82 C and the lower elastic area  82 D having superior elasticity extend, and the lower edge  82   c  protrudes significantly rearward with respect to the upper edge  82   a  of the lid-shaped airbag corresponding area  82 A, and thus the volume of inflation and development is ensured and the airbag  30  develops in a substantially triangular cross-sectional shape with which the energy absorbing stroke in a fore-and-aft direction of the vehicle body may be ensured. 
     The inflation and development of the airbag  30  are controlled and stable inflation and development are maintained by the elastic installation supporting members  27  and mainly the rear part  33  of the outer layer  31 , that expands by the rear part area  82 B surrounding the airbag corresponding area  82 A and the expandable elastic both side areas  82 C and lower elastic area  82 D. Thus, rearward movement, due to a rear-end collision, of the upper body Pb of the seated person Pf on the front seat  10  is elastically received by the airbag  30  and the rearward movement is restrained. 
     On the other hand, diagonally upwardly pushed knees Pn of the seated person Pr on the rear seat  60  are elastically received effectively by the lower portion of the airbag  30  which has significantly expanded and developed between the elastic installation supporting members  27  and the outer layer  31  in the seat back  20 , and thus forward movement is restrained and the upper body Pb of the seated person Pf on the front seat  10  avoids receiving the impact force F 1  from the knees Pn of the seated person Pr on the rear seat  60 . Similarly, the impact force F 1  from the knees and the impact force F 2  from the upper body of the seated person Pr moving forward on the rear seat  60  are elastically received and protected by the airbag  30  which inflates, develops and expands. 
     Another rear part  33 , which is of the outer layer  31  of the seat back  20  and expands along with the inflation and development of the airbag  30 , has an airbag corresponding area  83 A corresponding to the airbag  30  of the rear part  33  and a rear part area  83 B surrounding the airbag corresponding area  83 A as illustrated in  FIG. 11A , the airbag corresponding area  83 A being defined as a substantially U-shape along both side edges  83   b  and a upper edge  83   a , the rear part area  83 B having both side edges  83   e  and a upper edge  83   g , the both side edges  83   e  being gradually away from the both side edges  83   b  as being located downward from the lower end of each side edge  83   b , the upper edge  83   g  extending from the upper edge  83   a  with a predetermined distance apart and having both ends connected with the upper ends of the both side edges  83   e . The airbag corresponding area  83 A is formed in a substantially rectangular lid shape having a lower edge  83   c  which is connected with the rear part area  83 B. 
     A pair of elastic side areas  83 C and a strip-shaped upper elastic area  83 D are integrally formed, the elastic side areas  83 C being formed by placing a substantially downwardly tapered outer layer material superior in elasticity between the both side edges  83   b  of the airbag corresponding area  83 A and the both side edges  83   e  of the rear part area  83 B and by seaming the outer layer material and corresponding both side edges together, the upper elastic area  83 D being formed by placing the outer layer material between the upper edge  83   a  of the airbag corresponding area  83 A and the upper edge  83   g  of the rear part area  83 B and by seaming the outer layer material and corresponding upper edges together. 
     In this manner, the airbag  30  inflates and develops between the elastic installation supporting members  27  and the outer layer  31  due to a rear-end collision or the like. As illustrated in  FIG. 11B , when the airbag corresponding area  83 A is pressed from the inner side of the seat back including the rear part  33 , the elastic both side areas  83 C and the upper elastic area  83 D having superior elasticity extend, and the airbag  30  inflates and develops in a substantially triangular cross-sectional shape in which the upper edge  83   a  protrudes significantly rearward with respect to the lower edge  83   c  of the lid-shaped airbag corresponding area  83 A. 
     The inflation and development of the airbag  30  are controlled and stable inflation and development are maintained by the elastic installation supporting members  27  and mainly the rear part  33  of the outer layer  31 , that expands by the rear part area  83 B surrounding the airbag corresponding area  83 A and the expandable elastic both side areas  83 C and upper elastic area  83 D. Thus, rearward movement, due to a rear-end collision, of the upper body Pb of the seated person Pf on the front seat  10  is elastically received by the airbag  30  and the rearward movement is regulated. 
     On the other hand, diagonally upwardly pushed knees Pn of the seated person Pr on the rear seat  60  are elastically received effectively by the lower portion of the airbag  30  which has expanded and developed to be inclined between the elastic installation supporting members  27  and the outer layer  31  in the seat back  20 , and the upper body is elastically received effectively by the upper portion of the airbag  30  which inflates and develops significantly. The upper body Pb of the seated person Pf on the front seat  10  is protected against the impact force F 1  from the knees Pn of the seated person Pr on the rear seat  60 . Similarly, the impact force F 1  from the knees Pn and the impact force F 2  from the upper body of the seated person Pr moving forward on the rear seat  60  are elastically received and protected by the airbag  30  which inflates. 
     As illustrated in  FIG. 12A , another rear part  33 , which is of the outer layer  31  of the seat back  20  and expands along with the inflation and development of the airbag  30 , has an upper airbag corresponding area  84 A and a lower airbag corresponding area  84 B that correspond to the airbag  30  of the rear part  33 , and a rear part area  84 C, the upper and lower airbag corresponding areas  84 A,  84 B having both side edges  84   b  separated above and below and a lower edge  84 Ac and an upper edge  84 Ba separated across a central portion, a rear part area  84 C surrounding the upper airbag corresponding area  84 A and the lower airbag corresponding area  84 B, the rear part area  84 C having side edges  84   e  each being gradually away from the corresponding side edge  84   b  as being located near the center in a height direction from the lower end and the upper end of the side edge  84   b . The upper airbag corresponding area  84 A and the lower airbag corresponding area  84 B is formed in a substantially rectangular lid shape having an upper edge  84 Aa and a lower edge  84 Bc which are connected with the rear part area  84 C, the lower edge  84 Ac and the upper edge  84 Ba facing each other with a space. 
     A pair of elastic side areas  84 D and an elastic central area  84 E are integrally formed, the elastic side areas  84 D being formed by placing an outer layer material superior in elasticity between the side edges  84   b  of the upper and lower airbag corresponding areas  84 A,  84 B and the side edges  84   e  of the rear part area  84 C and by seaming the side edges  84   b  and the side edges  84   e  together, the elastic central area  84 E being formed by placing a strip-shaped outer layer material superior in elasticity between the lower edge  84 Ac of the upper airbag corresponding area  84 A and the upper edge  84 Ba of the lower airbag corresponding area  84 B and by seaming the lower edge  84 Ac and the upper edge  84 Ba together. Consequently, the inflation and development of the airbag  30  are controlled and stable inflation and development are maintained by the elastic installation supporting members  27 , and mainly the rear part area  84 C surrounding the upper airbag corresponding area  84 A and the lower airbag corresponding area  84 B, and the rear part  33  of the outer layer  31 , that expands by the expandable elastic side areas  84 D and elastic central area  84 E. 
     In this manner, the airbag  30  inflates and develops between the elastic installation supporting members  27  and the rear part  33  of the outer layer  31  due to a rear-end collision or the like. As illustrated in  FIG. 12B , when the upper airbag corresponding area  84 A and the lower airbag corresponding area  84 B are pressed from the inner side of the seat back including the rear part  33 , the elastic side areas  84 D and the elastic central area  84 E having superior elasticity extend, and the airbag  30  inflates and develops in a substantially trapezoidal cross-sectional shape in which the lower edge  84 Ac protrudes rearwardly with respect to the upper edge  84 Aa of the lid-shaped upper airbag corresponding area  84 A, the upper edge  84 Ba protrudes rearwardly with respect to the lower edge  84 Bc of the lower airbag corresponding area  84 B, and the upper airbag corresponding area  84 A and the lower airbag corresponding area  84 B are inclined. Thus, rearward movement, due to a rear-end collision, of the upper body Pb of the seated person Pf on the front seat  10  is elastically received by the airbag  30  and the rearward movement is regulated. 
     On the other hand, diagonally upwardly pushed knees Pn of the seated person Pr on the rear seat  60  are elastically received effectively by the lower portion of the airbag  30  which has expanded and developed to be inclined, and the upper body is elastically received effectively by the upper portion of the airbag  30  which inflates and develops, and forward movement is restrained, and thus the upper body Pb of the seated person Pf on the front seat  10  avoids receiving the impact force F 1  from the knees of the seated person Pr on the rear seat  60 . Similarly, the impact force F 1  from the knees Pn and the impact force F 2  from the upper body of the seated person Pr moving forward on the rear seat  60  are elastically received and protected by the airbag  30  which inflates, develops and expands. 
     The inflation and development of the airbag  30  are controlled and stable inflation and development are maintained by the elastic installation supporting members  27 , and mainly the rear part area  84 C surrounding the upper airbag corresponding area  84 A and the lower airbag corresponding area  84 B, and the rear part  33  of the outer layer  31 , that expands by the expandable elastic side areas  84 D and elastic central area  84 E. 
     Another rear part  33 , which is of the outer layer  31  of the seat back  20  and expands along with the inflation and development of the airbag  30 , has an airbag corresponding area  85 A corresponding to the airbag  30  of the rear part  33  and a rear part area  85 B surrounding the airbag corresponding area  85 A as illustrated in  FIG. 13A , the airbag corresponding area  85 A being defined as a substantially U-shape along both side edges  85   b  and a lower edge  85   c , the rear part area  85 B having both side edges  85   e  and a lower edge  85   f  along the side edges  85   b  and the lower edge  85   c  of the airbag corresponding area  85 A. The airbag corresponding area  85 A is formed in a substantially rectangular lid shape having an upper edge  85   a  which is connected with the rear part area  85 B. 
     A gusset  85 C folded in an upward tapered shape is placed between each side edge  85   b  of the airbag corresponding area  85 A and a corresponding side edge  85   e  of the rear part area  85 B, and the gusset and the side edges are seamed together. Similarly, a gusset  85 D folded in a strip form is placed between the lower edge  85   c  of the airbag corresponding area  85 A and the lower edge  85   f  of the rear part area  85 B, and the gusset and the lower edges are seamed together. Furthermore, the side edges  85   b  and the lower edge  85   c  of the airbag corresponding area  85 A, and the side edge  85   e  and the lower edge  85   f  of the rear part area  85 B are seamed together by a relatively fragile seam thread (not illustrated) with the corresponding edges overlapped or in contact with each other, the seam thread being breakable by the inflation and development of the airbag  30 . 
     In this manner, the airbag  30  inflates and develops between the elastic installation supporting members  27  and the rear part  33  of the outer layer  31  due to a rear-end collision or the like. As illustrated in  FIG. 13B , when the airbag corresponding area  85 A included in the rear part  33  is pressed, the pressure causes the seam thread to be broken, the seam thread being used for seaming the side edges  85   b  and the lower edge  85   c  of the airbag corresponding area  85 A, and the side edge  85   e  and the lower edge  85   f  of the rear part area  85 B together. Thus, the gusset  85 C and the gusset  85 D extend and develop, and the airbag  30  inflates and develops in a substantially triangular cross-sectional shape in which the volume of inflation and development is ensured such that the lower edge  85   c  protrudes significantly rearward with respect to the upper edge  85   a  of the lid-shaped airbag corresponding area  85 A. 
     In the inflation and development of the airbag  30 , development behavior and development pattern are controlled and stable inflation and development pattern are maintained by the elastic installation supporting members  27 , and mainly the rear part area  85 B surrounding the airbag corresponding area  85 A, and the expandable gusset  85 C and gusset  85 D. Thus, rearward movement, due to a rear-end collision, of the upper body Pb of the seated person Pf on the front seat  10  is elastically received by the airbag  30  and the rearward movement is regulated. 
     On the other hand, diagonally upwardly pushed knees Pn of the seated person Pr on the rear seat  60  are elastically received effectively by the lower portion of the airbag  30  which has significantly expanded and developed, and thus forward movement is restrained and the seated person Pf on the front seat  10  avoids receiving the impact force F 1  from the knees Pn of the seated person Pr on the rear seat  60 . 
     Similarly, the impact force F 1  from the knees Pn and the impact force F 2  from the upper body of the seated person Pr moving forward on the rear seat  60  are elastically received and protected by the airbag  30  which inflates, develops and expands. 
     Another rear part  33 , which is of the outer layer  31  of the seat back  20  and expands along with the inflation and development of the airbag  30 , has an airbag corresponding area  86 A corresponding to the airbag  30  of the rear part  33  and a rear part area  86 B surrounding the airbag corresponding area  86 A as illustrated in  FIG. 14A , the airbag corresponding area  86 A being defined as a substantially U-shape along both side edges  86   b  and an upper edge  86   a , the rear part area  86 B having both side edges  86   e  and an upper edge  86   d  along the side edges  86   b  and the upper edge  86   a  of the airbag corresponding area  86 A. The airbag corresponding area  86 A is formed in a substantially rectangular lid shape having a lower edge  86   c  which is connected with the rear part area  86 B. 
     A gusset  86 C folded in a downward tapered shape is placed between each side edge  86   b  of the airbag corresponding area  86 A and the corresponding side edge  86   e  of the rear part area  86 B, and the gusset and the side edges are seamed together. Similarly, a gusset  86 D folded in a strip form is placed between the upper edge  86   a  of the airbag corresponding area  86 A and the upper edge  86   d  of the rear part area  86 B, and the gusset and the lower edges are seamed together. Furthermore, the side edges  86   b  and the upper edge  86   a  of the airbag corresponding area  86 A, and the side edge  86   e  and the lower edge  86   d  of the rear part area  86 B are seamed together by a relatively fragile seam thread with the corresponding edges overlapped or in contact with each other, the seam thread being breakable by the inflation and development of the airbag  30 . 
     In this manner, the airbag  30  inflates and develops between the elastic installation supporting members  27  and the rear part  33  of the outer layer  31  due to a rear-end collision or the like. As illustrated in  FIG. 14B , when the airbag corresponding area  86 A included in the rear part  33  is pressed, the pressure causes the seam thread to be broken, the seam thread being used for seaming the side edges  86   b  and the upper edge  86   a  of the airbag corresponding area  86 A, and the side edge  86   e  and the upper edge  86   d  of the rear part area  86 B together. Thus, the gusset  86 C and the gusset  86 D extend and develop, and the airbag  30  inflates and develops in a substantially triangular cross-sectional shape in which the volume of inflation and development and the energy absorbing stroke are ensured such that the upper edge  86   a  protrudes significantly rearward with respect to the lower edge  86   c  of the lid-shaped airbag corresponding area  86 A. Thus, rearward movement, due to a rear-end collision, of the upper body Pb of the seated person Pf on the front seat  10  is elastically received by the airbag  30  and is regulated. 
     On the other hand, the seated person Pr on the rear seat  60  is elastically received effectively by the lower portion of the airbag  30  which has significantly expanded and developed, and thus forward movement is restrained and the seated person Pf on the front seat  10  avoids receiving the impact force F 1  from the knees Pn of the seated person Pr on the rear seat  60 . Similarly, the impact force F 1  from the knees Pn and the impact force F 2  due to contact of the upper body of the seated person Pr moving forward on the rear seat  60  are elastically received and protected by the airbag  30  which inflates, develops and expands. 
     Another rear part  33 , which is of the outer layer  31  of the seat back  20  and expands along with the inflation and development of the airbag  30 , has an airbag corresponding area  87 A corresponding to the airbag  30  of the rear part  33  and a rear part area  87 B surrounding the airbag corresponding area  87 A as illustrated in  FIG. 15A , the airbag corresponding area  87 A being partitioned by a fragile portion  87   d  which is a substantially U-shape along both side edges  87   b  and a lower edge  87   c  and which serves as a rupture portion which is breakable by the inflation and development of the airbag  30 . The airbag corresponding area  87 A is formed in a substantially rectangular lid shape having an upper edge  87   a  which is connected with the rear part area  87 B. 
     In this manner, the airbag  30  inflates and develops between the elastic installation supporting members  27  and the rear part  33  of the outer layer  31  due to a rear-end collision or the like. As illustrated in  FIG. 15B , when the airbag corresponding area  87 A included in the rear part  33  is pressed, the pressure causes the fragile portion  87   d  to be gradually ruptured and opened from below, and the airbag  30  inflates and develops in a substantially triangular cross-sectional shape in which the volume of inflation and development and the energy absorbing stroke are ensured such that the lower edge  87   c  of the airbag corresponding area  87 A protrudes significantly rearward, the fragile portion  87   d  being formed along the area from the lower edge  87   c  to the both side edges  87   b.    
     The inflation and development of the airbag  30  are controlled and thus stable inflation and development are maintained by the elastic installation supporting members  27  and the rear part area  87 B surrounding the airbag corresponding area  87 A which is ruptured in the fragile portion  87   d  along with the inflation and development of the airbag  30 . Thus, rearward movement, due to a rear-end collision, of the upper body Pb of the seated person Pf on the front seat  10  is elastically received by the airbag  30  and the rearward movement is regulated. 
     On the other hand, diagonally upwardly pushed knees Pn of the seated person Pr on the rear seat  60  are elastically received effectively by the lower portion of the airbag  30  which has expanded and developed, and thus forward movement is restrained and the upper body Pb of the seated person Pf on the front seat  10  avoids receiving the impact force F 1  from the knees Pn of the seated person Pr on the rear seat  60 . Similarly, the impact force F 1  due to contact of the knees and the impact force F 2  due to contact of the upper body of the seated person Pr on the rear seat  60  are elastically received and protected by the airbag  30  which inflates, develops and expands. 
     Another rear part  33 , which is of the outer layer  31  of the seat back  20  and expands along with the inflation and development of the airbag  30 , has an airbag corresponding area  88 A corresponding to the airbag  30  of the rear part  33  and a rear part area  88 B surrounding the airbag corresponding area  88 A as illustrated in  FIG. 16A , the airbag corresponding area  88 A being partitioned by a fragile portion  88   d  which is a substantially U-shape along both side edges  88   b  and an upper edge  88   a  and which serves as a rupture portion which is breakable by the inflation and development of the airbag  30 . The airbag corresponding area  88 A is formed in a substantially rectangular lid shape having a lower edge  88   c  which is connected with the rear part area  88 B. 
     In this manner, the airbag  30  inflates and develops between the elastic installation supporting members  27  and the rear part  33  of the outer layer  31  due to a rear-end collision or the like. As illustrated in  FIG. 16B , when the airbag corresponding area  88 A included in the rear part  33  is pressed, the pressure causes the fragile portion  88   d  to be gradually ruptured and opened from above, and the airbag  30  inflates and develops in a substantially triangular cross-sectional shape in which the volume of inflation and development and the energy absorbing stroke are ensured such that the upper edge  88   a  of the airbag corresponding area  88 A protrudes significantly rearward, the fragile portion  88   d  being formed along the area from the upper edge  88   c  to the both side edges  88   b.    
     The inflation and development of the airbag  30  are controlled and stable inflation and development are maintained by the elastic installation supporting members  27  and the rear part area  88 B surrounding the airbag corresponding area  88 A which is divided via the fragile portion  88   d  that is ruptured along with the inflation and development of the airbag  30 . Thus, rearward movement, due to a rear-end collision, of the upper body Pb of the seated person Pf on the front seat  10  is elastically received by the airbag  30  and the rearward movement is regulated. 
     On the other hand, diagonally upwardly pushed knees Pn of the seated person Pr on the rear seat  60  are elastically received by the lower portion of the airbag  30  and forward movement is restrained, whereas the upper body such as the chest is elastically received by the upper portion of the airbag  30  which has significantly expanded and developed, and forward movement is restrained. 
     In this manner, the upper body Pb of the seated person Pf on the front seat  10  avoids receiving the impact force F 1  from the knees Pn of the seated person Pr on the rear seat  60 . Similarly, the impact force F 1  due to contact of the knees and the impact force F 2  from the upper body of the seated person Pr moving forward on the rear seat  60  are elastically received and protected by the airbag  30  which inflates, develops and expands. 
     Another rear part  33 , which is of the outer layer  31  of the seat back  20  and expands along with the inflation and development of the airbag  30 , has an upper airbag corresponding area  89 A and a lower airbag corresponding area  89 B that correspond to the airbag  30  of the rear part  33 , and a rear part area  89 C surrounding the upper airbag corresponding area  89 A and the lower airbag corresponding area  89 B, the upper airbag corresponding area  89 A, the lower airbag corresponding area  89 B, and the rear part area  89 C being partitioned in a substantially H-shape by fragile portions  89   c ,  89   d  which are breakable by the inflation and development of the airbag  30 , the substantially H-shape extending in a width direction at the center of both side edges  89   b ,  89   b  in a height direction. A lower edge  89 Ac of the upper airbag corresponding area  89 A and an upper edge  89 Ba of the lower airbag corresponding area  89 B are connected via the fragile portion  89 . The upper airbag corresponding area  89 A and the lower airbag corresponding area  89 B are formed in a substantially rectangular lid shape respectively having an upper edge  89 Aa and a lower edge  89 Bc which are connected with the rear part area  89 C. 
     In this manner, the airbag  30  inflates and develops between the elastic installation supporting members  27  and the rear part  33  of the outer layer  31  due to a rear-end collision or the like. As illustrated in  FIG. 17B , when the upper airbag corresponding area  89 A and the lower airbag corresponding area  89 B forming the rear part  33  are pressed, the pressure causes the fragile portion  89   d  and the fragile portions  89   c  to be gradually ruptured, the fragile portion  89   d  being between the upper airbag corresponding area  89 A and the lower airbag corresponding area  89 B, the fragile portions  89   c  being formed along the both side edges  89   b.    
     Then the space between the lower edge  89 Ac of the upper airbag corresponding area  89 A and the upper edge  89 Ba of the lower airbag corresponding area  89 B starts to be opened, and the airbag  30  inflates and develops in a substantially trapezoidal or triangular shape in which the volume of inflation and development and the energy absorbing stroke are available such that the lower edge  89 Ac of the upper airbag corresponding area  89 A and the upper edge  89 Ba of the lower airbag corresponding area  89 B at the center in a height direction protrude significantly rearward. 
     The inflation and development of the airbag  30  are controlled and stable inflation and development are maintained by the elastic installation supporting members  27 , the upper airbag corresponding area  89 A, the lower airbag corresponding area  89 B, and the rear part area  89 C surrounding the upper airbag corresponding area  89 A and the lower airbag corresponding area  89 B, the upper airbag corresponding area  89 A and the lower airbag corresponding area  89 B being ruptured in the fragile portions  89   d ,  89   c  along with the inflation and development of the airbag  30 . Thus, rearward movement, due to a rear-end collision, of the upper body Pb of the seated person Pf on the front seat  10  is elastically received by the airbag  30  and the rearward movement is restrained. 
     On the other hand, diagonally upwardly pushed knees Pn of the seated person Pr on the rear seat  60  are elastically received by the lower portion of the airbag  30  which has expanded and developed between the elastic installation supporting members  27  and the rear part  33  of the outer layer  31  in the seat back  20 , and forward movement is restrained, whereas the upper body such as the chest is elastically received by the upper portion of the airbag  30  which has significantly expanded and developed, and forward movement is restrained. 
     In this manner, the upper body Pb of the seated person Pf on the front seat  10  avoids receiving the impact force F 1  from the knees Pn of the seated person Pr on the rear seat  60 , and the seated person Pf on the front seat  10  is protected. Similarly, the impact force F 1  from the knees and the impact force F 2  from the upper body of the seated person Pr moving forward on the rear seat  60  are elastically received and protected by the airbag  30  which inflates, develops and expands. 
     In each of the above-described implementations, the rear board may be disposed along an expansion area which is formed in the rear part of the outer layer  31 , and thus disposition of the rear board has no effect on the expansion deformation of the expansion area. 
     Thus, according to the above-described implementations, the simple configuration, in which the airbag disposed within the seat back causes the outer layer to expand rearward within the seat back, ensures the volume of inflation and development and the energy absorbing stroke of the airbag, thereby making it possible to elastically receive rearward movement of a seated person due to a collision and to protect the seated person against an impact from the rear. Consequently, without adopting a complicated configuration, direct protection of a seated person by the airbag at the time of a collision and accurate reduction of an impact from the rear of the seat back may be achieved. 
     Third Implementation 
     A third implementation will be described with reference to  FIGS. 18 to 22 .  FIG. 18  is a vertical cross-sectional view illustrating the outline of a vehicle seat;  FIG. 19  is a schematic perspective view with partial cutaway of the vehicle seat as seen from the rear;  FIG. 20A  is an enlarged view of portion a of  FIG. 18  illustrating the outline of a rear board supporter;  FIG. 20B  is an enlarged view of portion b of  FIG. 18  illustrating the outline of a rear board retainer; and  FIG. 21  is an operation explanatory diagram of the vehicle seat when an impact occurs. It is to be noted that the components corresponding to those in the first implementation are denoted by the same symbols and detailed description of the components is omitted. 
     As illustrated in  FIGS. 18 and 19 , in the front seat  10 , a plurality of elastic installation supporting members  27  is provided in the seat back frame  21  that forms the framework of the seat back  20 , the seat pad  28  is disposed on the front surface of the elastic installation supporting members  27 , the airbag  30  in a contracted state is disposed between the elastic installation supporting members  27  and a rear board  90 , and the entire components except for the rear board  90  are covered by the outer layer  31  in a bag shape. 
     The seat pad  28 , which elastically supports the upper body Pb of the seated person Pf from the rear part side, is disposed on the front surface of the elastic installation supporting members  27 , the seat pad being composed of a urethane foam material. The airbag  30 , which is flatly folded in a rectangular shape in a contracted state, is disposed between the elastic installation supporting members  27  and the rear board  90 . 
     The rear board  90  is in a substantially rectangular shape and covers the rear part of the seat back  20  in the rear of the airbag  30 . The upper portion and the lower portion of the rear board  90  are attached to the seat back frame  21  by a rear board supporter  91  and a rear board retainer  92 , respectively. It is to be noted that the rear board  90  is made of a hard resin and includes a plurality of fragile portions  90   a  that are each formed as a groove or a thin layer to achieve predetermined bending deformation. Also, due to the formation of the fragile portions  90   a , when an excessive load is applied to the rear board  90 , the rear board  90  bends along the fragile portions  90   a  or are ruptured along the fragile portions  90   a  and thus fracture of unspecified portions is prevented and at the time of fracture, occurrence of sharp protrusions on a fracture surface, that is, occurrence of sharp edges may be avoided. 
     As illustrated in  FIGS. 18 and 20A , the rear board supporter  91  includes a supporting bracket  91 A, a bracket  91 B, and a hinge mechanism  91 C, the supporting bracket being connected to the upper end rear face of the right and left side brackets  24  and protruding rearward, the bracket being connected to the upper both ends of the rear board  90  and protruding forward, the hinge mechanism connecting the supporting bracket and bracket in a swingable manner. In this manner, the upper portion of the rear board  90  is supported by the upper portion of the seat back frame  21  via the hinge mechanism  91 C as a fulcrum in a forwardly and rearwardly swingable manner. 
     As illustrated in  FIGS. 18 and 20B , the rear board retainer  92  is a hat-like shape which is attached to the lower end rear face of each side bracket  24 , and includes a retaining bracket  92 A, a clip holder  92 B, and a clip  92 C, the retaining bracket including a clip retaining hole  92   a  on a portion of the rear board  90  in a protruding manner, the clip being mounted and supported on the end of the clip holder. The clip  92 C mounted and supported on the clip retainer  92 B is inserted and retained in the clip retaining hole  92   a  of the retaining bracket  92 A provided in the side bracket  24 , thereby fixing the rear board  90  at a normal use position illustrated in  FIG. 18 . 
     On the other hand, when a load greater than or equal to a predetermined value is applied to the front surface of the rear board  90  from the front, as illustrated in  FIGS. 18 and 20B  by a virtual line, along with rearward movement of the rear board  90 , the clip  92 C mounted and supported on the clip retainer  92 B is removed from the clip retaining hole  92   a  of the retaining bracket  92 A and the fixing is released. 
     On the other hand, when the airbag  30  inflates and develops between the elastic installation supporting members  27  and the rear board  90  due to expanded gas injection of the inflator  29 , as illustrated in  FIG. 21 , the airbag  30  pushes forward and urges the elastic installation supporting members  27  and the seat pad  28  so as to allow the upper body Pb of the seated person Pf to be elastically supported from the rear side, and the airbag  30  also pushes the rear board  90  rearward to remove the clip  92 C mounted and supported on the clip retainer  92 B from the clip retaining hole  92   a  of the retaining bracket  92 A, and thus the fixing is released. In addition, the rear board  90  is moved rearward, that is, the rear board  90  is swung rearward, thereby ensuring the volume of inflation and development of the airbag  30  and the energy absorbing stroke of the airbag  30  in a fore-and-aft direction of the vehicle body. The expandable and developable airbag  30  allows an impact force applied to the rear board  90  to be elastically received. 
     The inflation and development of the airbag  30  are sandwiched and held by the elastic installation supporting members  27  that hold the airbag  30  and a wide area in the swingable rear board  90 , and thus development behavior and development pattern are controlled and stable inflation and development are maintained. Also, the inflatable and developable airbag  30  is supported by the swingable rear board  90  from the rear part side, and inflates and develops two-dimensionally, and uniform pressure is thereby applied to a wide area in the upper body Pb of the seated person Pf. 
     In a vehicle equipped with the vehicle seat configured in this manner, when inevitability of a rear-end collision of the vehicle is predicted based on collision occurrence information from the collision detection sensor, a drive signal is outputted from the control unit to the inflator  29  at a predicted occurrence time of collision, the inflator  29  is ignited and expanded gas jets from the inflator  29 , and the airbag  30  inflates and develops between the elastic installation supporting members  27  and the rear board  90 . 
     As illustrated in  FIG. 21 , the inflated and developed airbag  30  pushes the elastic installation supporting members  27  and the seat pad  28  forward as well as pushes the rear board  90  rearward, and thus the clip  92 C supported on the clip retainer  92 B is removed from the clip retaining hole  92   a  of the retaining bracket  92 A, thereby causing the rear board  90  to swing. 
     Thus, rearward movement, due to a rear-end collision, of the upper body Pb of the seated person Pf on the front seat  10  is elastically received by the airbag  30  that inflates and develops via the outer layer  31 , the seat pad  28 , and the elastic installation supporting members  27 , and so the rearward movement is regulated. 
     On the other hand, the seated person Pr on the rear seat  60  is pushed forward and moved due to vehicle body deformation by an impact of a rear-end collision or rebounding after being moved rearward and pressed against the seat back, and for example, the knees Pn may come into contact with the rear board  90  which is caused to be swung by inflation and development of the airbag  30 . The impact force F 1  at the contact of the knees Pn with the rear board  90  of the front seat  10  is elastically received by a wide area in the airbag  30  via the rear board  90 , and thus the upper body Pb of the seated person Pf on the front seat  10  avoids receiving the impact force F 1  from the knees Pn of the seated person Pr on the rear seat  60 , thereby protecting the seated person Pf on the front seat  10 . Similarly, the impact force F 1  from the knees Pn and the impact force from the upper body of the seated person Pr moving forward on the rear seat  60  are elastically received and protected by the airbag  30  for which the volume of inflation and development is ensured. 
     Particularly, when the knees Pn of the seated person Pr on the rear seat  60  come into contact with the rear part of the rear board  90  in an inclined manner, the contacting knees Pn are slid and moved along the rear part of the rear board  90 , thereby reducing the impact force F 1  from the knees in contact with the rear board  90 . For this reason, the rear board  90  may be disposed to be inclined with respect to the fore-and-aft direction of the vehicle body. 
     The above-described implementations each have a configuration in which fixing of the rear board  90  by the rear board retainer  92  is released by pressing movement of the rear board  90  caused by the expanded and developed airbag  30 . However, when the airbag  30  inflates and develops, fixing of the rear board  90  is released beforehand so as to be swingable and thus fixing releasing operation utilizing the inflation and development of the airbag  30  is eliminated and load to the airbag  30  that inflates and develops may be reduced. 
     Specifically, for example, instead of the rear board retainer  92  that fixes the seat back frame  21  and the rear board  90  together, a rear board retainer  93 , which serves as a fixing releasing mechanism, is provided as illustrated in  FIG. 22 . The rear board retainer  93  includes a pair of retaining brackets  93 A and a locking mechanism  93 B, the retaining brackets having retaining holes  93   a  provided in the lower rear face of the side brackets  24  of the seat back frame  21 , the locking mechanism for selectively moving a shaft  93   b  to a locked position at which the shaft is inserted through the retaining holes  93   a  or an unlocked position at which the shaft is retracted from the retaining holes by an electromagnetic actuator  93 Ba. On the other hand, the lower end of the rear board  90  is provided with a retaining portion  93 C which includes a through hole  93   c  and is insertable in between the pair of retaining brackets  93 A at the normal use position of the rear board  90 , the retaining brackets being provided in the seat back frame  21 . 
     In this configuration, the rear board  90  is fixed at the normal use position by inserting the retaining portion  93 C provided at the rear board  90  into between both retaining brackets  93 A provided at the seat back frame  21 , and setting the shaft  93   b  of the locking mechanism  93 B at the locked position by inserting the shaft  93   b  into the through hole  93   c  of the retaining portion  93 C. On the other hand, by moving the shaft  93   b  of the locking mechanism  93 B to the unlocked position, the shaft  93   b  is retracted from the through hole  93   c  and fixing is released. 
     When inevitability of a rear-end collision of a vehicle is predicted based on collision occurrence information from the collision detection unit, a drive signal from the control unit at a predicted occurrence time of collision causes the electromagnetic actuator  93 Ba to operate to move the shaft  93   b  to the unlocked position so that the rear board  90  becomes swingable to allow the airbag  30  to inflate and develop. Thus load to inflation and development of the airbag  30  is reduced and the inflation and development of the airbag  30  may be performed more smoothly and quickly. 
     It is to be noted that the rear board supporter  91  that causes the rear board  90  to be supported on the seat back frame  21 , the rear board retainer  92 , and the rear board retainer  93  serving as a fixing releasing mechanism are not limited to the above-described configuration, and may be formed by another suitable configuration having the above-described functions. 
     Fourth Implementation 
     A fourth implementation will be described with reference to  FIGS. 23 to 25 .  FIG. 23  is a vertical cross-sectional view illustrating the outline of a vehicle seat;  FIG. 24A  is an enlarged perspective view of portion c of  FIG. 23  illustrating the outline of a rear board supporter and  FIG. 24B  is an enlarged view of portion d of  FIG. 23  illustrating the outline of a rear board retainer; and  FIG. 25  is an operation explanatory diagram of a vehicle seat when an impact occurs. It is to be noted that the vehicle seat in the present implementation differs from the vehicle seat in the third implementation in mounting configuration of the rear board  90 , and other major configurations are the same as those in the third implementation. The components in  FIGS. 23 to 25  corresponding to those in  FIGS. 18 to 22  are denoted by the same symbols and detailed description of the components is omitted and different configurations will be mainly described. 
     As illustrated in  FIG. 23 , in the rear of the airbag  30 , the upper portion of the rear board  90  disposed on the rear of the seat back  20  is supported on an upper part of the seat back frame  21  by a rear board supporter  95 , and the lower portion of the rear board  90  is supported on a lower part of the seat back frame  21  by a rear board retainer  96 . 
     As illustrated in  FIG. 24A  which is an enlarged perspective view of the portion c of  FIG. 23 , the rear board supporter  95  is a hat-like shape which is connected to the upper rear face of the right and left side brackets  24 , and includes a supporting bracket  95 A and an engagement bracket  95 B, the supporting bracket having a recessed groove-shaped retaining groove  95   a  at the lower end, the engagement bracket protruding forward, being connected to upper both ends of the rear board  90  and having a recessed groove-shaped engagement part  95   b  which may be locked in the retaining groove  95   a  of the supporting bracket  95 A from below. 
     By engaging the retaining groove  95   a  of the supporting bracket  95 A with the engagement part  95   b  of the engagement bracket  95 B, the upper portion of the rear board  90  is held at the normal use position, and downward movement of the rear board  90  causes the engagement part  95   b  of the engagement bracket  95 B to be removed from the retaining groove  95   a  of the supporting bracket  95 A, and thus the engagement is released. 
     As illustrated in  FIG. 24B  which is an enlarged perspective view of the portion d of  FIG. 23 , a rear board retainer  96  is a hat-like shape which is attached to the lower end rear face of each side bracket  24 , and includes a retaining bracket  96 A, a clip holder  96 B, and a clip  96 C, the retaining bracket including a clip retaining hole  96   a  on the rear end face, the clip holder being formed in the lower both ends of the rear board  90  in a protruding manner, the clip being mounted and supported on the end of the clip holder. 
     The clip  96 C mounted and supported on the clip retainer  96 B is inserted and retained in the clip retaining hole  96   a  of the retaining bracket  96 A provided in the side bracket  24 , thereby fixing the lower portion of the rear board  90  at the normal use position. On the other hand, when a load greater than or equal to a predetermined value is applied to the front surface of the rear board  90  from the front, along with rearward movement of the rear board  90 , the clip  96 C supported on the clip retainer  96 B is removed from the clip retaining hole  96   a  of the retaining bracket  96 A, and thus fixing of the rear board  90  is released. 
     The rear board  90  is fixed at the normal use position of the rear of the seat back  20  by engaging the engagement part  95   b  of the engagement bracket  95 B with the retaining groove  95   a  of the supporting bracket  95 A from below and by inserting and retaining the clip  96 C mounted and supported on the clip retainer  96 B into the clip retaining hole  96   a  of the retaining bracket  96 A, the engagement bracket  95 B being provided in the upper portion of the rear board  90 , the supporting bracket  95 A being provided in the upper portion of the side bracket  24 , the clip retainer  96 B being provided in the rear board  90 , the retaining bracket  96 A being provided in the side bracket  24 . 
     When a load greater than or equal to a predetermined value is applied to the front surface of the rear board  90  from the front, along with rearward movement of the rear board  90 , the clip  96 C supported on the clip retainer  96 B is removed from the clip retaining hole  96   a  of the retaining bracket  96 A, and the movement of the rear board  90  causes the engagement part  95   b  of the engagement bracket  95 B to be removed from the retaining groove  95   a  of the supporting bracket  95 A, and thus the engagement is released, which allows the rear board  90  to be detached from the rear of the seat back  20 . 
     On the other hand, when the airbag  30  inflates and develops between the elastic installation supporting members  27  and the rear board  90  due to expanded gas injection of the inflator  29 , as illustrated in  FIG. 25 , the airbag  30  pushes forward and urges the elastic installation supporting members  27  and the seat pad  28  so as to allow the upper body Pb of the seated person Pf to be elastically supported from the rear side, and the movement of the rear board  90  causes the engagement part  95   b  of the engagement bracket  95 B to be removed from the retaining groove  95   a  of the supporting bracket  95 A, and thus the engagement is released, which allows the rear board  90  to be detached from the rear of the seat back  20 . Consequently, the inflation and development of the airbag  30  are sandwiched and held by the elastic installation supporting members  27  that hold the airbag  30  and a wide area in the rear board  90 , and thus development behavior and development pattern are controlled and stable inflation and development pattern are maintained, and the amount of inflation and development and the energy absorbing stroke in the fore-and-aft direction of the vehicle body are ensured. Also, the airbag  30  is supported by the detached rear board  90  from the rear part side, and inflates and develops two-dimensionally, and uniform pressure is thereby applied to a wide area in the upper body Pb of the seated person Pf. 
     In a vehicle equipped with the vehicle seat configured in this manner, when inevitability of a rear-end collision of the vehicle is predicted based on collision occurrence information from the collision detection unit, a drive signal is outputted from the control unit to the inflator  29  at a predicted occurrence time of collision, expanded gas jets from the inflator  29 , and the airbag  30  inflates and develops between the elastic installation supporting members  27  and the rear board  90 . 
     The inflated and developed airbag  30  pushes the elastic installation supporting members  27  and the seat pad  28  forward, whereas the clip  96 C supported on the clip retainer  96 B is removed from the clip retaining hole  96   a  of the retaining bracket  96 A, and the movement of the rear board  90  causes the engagement part  95   b  of the engagement bracket  95 B to be removed from the retaining groove  95   a  of the supporting bracket  95 A, and thus the engagement is released and the rear board  90  moves rearward. 
     Thus, rearward movement, due to a rear-end collision, of the upper body Pb of the seated person Pf on the front seat  10  is elastically received by the airbag  30  that inflates and develops via the outer layer  31 , the seat pad  28 , and the elastic installation supporting members  27 , and so the rearward movement is regulated. 
     On the other hand, when the rear board  90  is pushed rearward by the inflation and development of the airbag  30 , the seated person Pr on the rear seat  60  may come into contact with the rear board  90  of the front seat  10 , for example, at the knees Pn. The impact force F 1  at the contact of the knees Pn with the rear board  90  of the front seat  10  is elastically received by a wide area in the airbag  30  via the rear board  90 , and thus the upper body Pb of the seated person Pf on the front seat  10  avoids receiving the impact force F 1  from the knees Pn of the seated person Pr on the rear seat  60 , thereby protecting the seated person Pf on the front seat  10 . Similarly, the impact force F 1  from the knees Pn and the impact force from the upper body of the seated person Pr moving forward on the rear seat  60  are elastically received and protected by the airbag  30  for which the volume of inflation and development and the energy absorbing stroke are ensured. 
     Although the above-described implementations each have a configuration in which fixing of the rear board  90  with the rear board retainer  96  is released by the pressure caused by the inflation and development of the airbag  30 , instead of the rear board retainer  96 , for example, a rear board retainer similar to the rear board retainer  93  illustrated in  FIG. 22  may be provided. When the airbag  30  inflates and develops at a predicted occurrence time of collision, releasing the fixing of the rear board  90  beforehand allows the rear board  90  to be moved, and thus it is unnecessary to release the fixing of the rear board  90  by the movement of the rear board  90  which is caused by the airbag  30 , and thus load to the airbag  30  at the inflation and development may be reduced. 
     Fifth Implementation 
     A fifth implementation will be described with reference to  FIGS. 26 and 27 . It is to be noted that  FIGS. 26 and 27  correspond to  FIGS. 18 and 21 , and the present implementation differs from the third implementation in mounting configuration of the rear board  90 , and other major configurations are the same as those in the third implementation. The components in  FIGS. 26 and 27  corresponding to those in  FIGS. 18 and 21  are denoted by the same symbols and detailed description of the components is omitted and different configurations will be mainly described. 
     As illustrated in  FIG. 26 , in the outer layer  31  of the seat back  20 , the airbag  30  is covered by the elastic rear part  33 , and the rear board  90  is disposed in the rear of the rear part  33 . The rear board  90  has a rectangular tubular shape that covers the airbag  30 , and includes fragile portions  90   a  which are each a groove or a thin layer to facilitate predetermined bending deformation. 
     When the airbag  30  inflates and develops two-dimensionally between the elastic installation supporting members  27  and the rear part  33  of the outer layer  31  due to expanded gas injection of the inflator  29 , as illustrated in  FIG. 27 , the airbag  30  pushes forward and urges the elastic installation supporting members  27  and the seat pad  28 , and causes the rear part  33  of the outer layer  31  and the rear board  90  to expand so as to sufficiently ensure the volume of inflation and development of the airbag  30  and the energy absorbing stroke of the airbag  30 . The inflation and development of the airbag  30  are sandwiched and held by the elastic installation supporting members  27  that hold the airbag  30 , and extending, expanding rear part  33  of the outer layer  31  and the rear board  90 , and thus development behavior and development pattern are controlled and stable inflation and development pattern is maintained. Also, the airbag  30  is supported by the rear board  90  from the rear part side, and inflates and develops two-dimensionally, and uniform pressure is thereby applied to a wide area in the upper body Pb of the seated person Pf. 
     In a vehicle equipped with the vehicle seat configured in this manner, when inevitability of a rear-end collision of the vehicle is predicted based on signals from the collision detection unit, a drive signal is outputted from the control unit to the inflator at a predicted occurrence time of collision, expanded gas jets from the inflator, and the airbag  30  inflates and develops between the elastic installation supporting members  27  and the rear part  33  of the outer layer  31  as illustrated in  FIG. 27 . Thus, rearward movement, due to a rear-end collision, of the upper body Pb of the seated person Pf on the front seat  10  is elastically received by the airbag  30  via the outer layer  31 , the seat pad  28 , and the elastic installation supporting members  27 , and so the rearward movement is regulated. 
     On the other hand, the seated person Pr on the rear seat  60  may be moved forward and, for example, the knees Pn may come into contact with the rear board  90  of the front seat  10 . At this point, the knees Pn are elastically received via the outer layer  31  and the rear board  90  by the airbag  30  which has expanded and developed between the elastic installation supporting members  27 , and the rear part  33  of the outer layer  31 , the rear board  90 , and thus forward movement is restrained and the upper body Pb of the seated person Pf on the front seat  10  avoids receiving the impact force F 1  from the knees Pn of the seated person Pr on the rear seat  60 . Similarly, the knees Pn and the upper body of the seated person Pr moving forward on the rear seat  60  are elastically received and protected by the airbag  30  which has inflated and developed. 
     Also when the rear board  90  is damaged due to an impact, the airbag  30  is held by the outer layer  31  and the development behavior and development pattern of the inflation and development of the airbag  30  are controlled, and stable inflation development pattern is maintained. 
     Thus, according to each of the third to fifth implementations described above, with a simple configuration in which the airbag disposed between the seat pad and the rear board within the seat back inflates and develops to push the rear board rearward, the airbag, which is supported by the rear board and inflates and develops, makes it possible to elastically receive rearward movement of a seated person due to a collision by uniform pressure over a wide area and to protect the seated person against an impact from the rear, thereby enabling direct protection of a seated person by the airbag at the time of a collision and accurate reduction of an impact from the rear of the seat back. 
     In the above description, the case is taken as an example in which the airbag  30  is caused to inflate and develop by a single inflator. However, the airbag  30  may be caused to inflate and develop in multiple steps in a suitable manner to a seated person. For example, the airbag  30  is caused to inflate and develop in multiple steps using a plurality of inflators in a suitable manner to a seated person so that the development state of the airbag  30 , such as an inflation development pressure and a development time may also be controlled. 
     Sixth Implementation 
     A sixth implementation will be described with reference to  FIGS. 28 and 29 .  FIG. 28  is a schematic rear view with partial cutaway illustrating the outline of the present implementation; and  FIG. 29  is a cross-sectional view of  FIG. 28  taken along line XXIX-XXIX. It is to be noted that the components corresponding to those in the first implementation are denoted by the same symbols as in  FIGS. 1 to 5  and detailed description of the components is omitted. 
     As illustrated in  FIGS. 28 and 29 , the inflator  29  is attached to a side frame  23  or a side bracket  24  outwardly in the vehicle body so as to be located inwardly of the seat back frame  21 . As illustrated in  FIG. 29 , the inflator  29  is disposed so as to be located forward of the airbag  30  with the elastic installation supporting members  27  interposed therebetween in the fore-and-aft direction of the vehicle body. 
     With this configuration, in the case where the airbag  30  inflates and develops within the seat back  20  due to a collision, the reactive force is received by the elastic installation supporting members  27 , and thus prevention effect on interference between the airbag  30  and the inflator  29  is obtained. 
     Seventh Implementation 
     A seventh implementation will be described with reference to  FIGS. 30 to 32 .  FIG. 30  is a schematic cross-sectional view of the front seat  10 ;  FIG. 31  is a schematic perspective view with partial cutaway of the front seat  10  as seen from the rear; and  FIG. 32  is an operation explanatory diagram of the front seat  10  when an impact occurs. It is to be noted that the components corresponding to those in the first implementation are denoted by the same symbols as in  FIGS. 1 to 5  and detailed description of the components is omitted. In the present implementation, the inflator  29  is not attached to a side bracket  24 . As illustrated in  FIGS. 30 and 31 , the inflator  29  is disposed in parallel to the upper cross member  25  and fixed thereto so that the circumferential surface of the inflator  29  is in contact with substantially the central position of the upper cross member  25  in the extending direction. Therefore, the inflator  29  is attached within the seat back frame  21  and to an upper portion of the seat back frame  21 . 
     In the case where another vehicle collides with a vehicle having the front seat  10 , the inflator  29  is ignited by an operation of a collision detection unit or collision information predicted by a collision prediction unit, and expanded gas from the inflator  29  causes the airbag  30  to instantly inflate and develop between the elastic installation supporting members  27  and the rear surface of the seat back  20  as illustrated in  FIG. 32 . 
     In this process, the front of the airbag  30  is covered with flexible materials such as the elastic installation supporting member  27 , the seat pad  28 , and the outer layer  31 , and thus due to the inflation and development of the airbag  30 , as illustrated in  FIG. 32 , these flexible materials exhibit slight deformation in the forward direction of the vehicle body, whereas the outer layer  31 , which covers the rear of the airbag  30  as a flexible material, exhibits significantly protruding deformation in the rearward direction of the vehicle body. 
     With the front seat  10 , the deformation behavior of the flexible members and flexible materials in front and in rear of the airbag  30  allows the volume E of development of the airbag  30  in the fore-and-aft direction of the vehicle body to be ensured, and also allows the energy absorbing stroke of the seat back  20  in the fore-and-aft direction of the vehicle body to be sufficiently ensured. 
     That is, as illustrated in  FIG. 32 , rearward movement, due to an impact of a rear-end collision, of the upper body Pb of the front seat person Pf is elastically received by the outer layer  31  at the front of the seat back  20 , the seat pad  28 , the elastic installation supporting members  27 , and the inflated and developed airbag  30 , and thus the upper body Pb of the front seat person Pf is protected against the impact to be received when being pressed against the seat back  20 . 
     On the other hand, due to an impact of a rear-end collision, the rear seat person Pr is pressed against the rear seat  60 , and is moved forward by subsequent rebound. Due to the rebound, the knees Pn of the rear seat person Pr moving forward is elastically received by the outer layer  31  on the rear of the seat back and the airbag  30 , and thus the upper body Pb of the seated person Pf on the front seat  10  is protected against the collision impact of the knees Pn of the rear seat person Pr. 
     In this process, the airbag  30  is restrained to some extent by the internal geometry within the seat back  20  located immediately rearward of the upper body Pb of the seated person Pf on the front seat  10 , and development behavior and development pattern are controlled and the airbag  30  is caused to stably inflate and develop. 
     Consequently, rearward movement of the upper body Pb of the seated person Pf may be reliably received, and the impact due to forward movement of the knees Pn of the rear seat person Pr may be reliably absorbed, and thus proper protection for the seated person Pf may be achieved. 
     In addition, such protection of the seated person Pf is achieved by the airbag  30  that inflates and develops within the seat back  20  and disposition of the flexible materials in front and in rear of the airbag  30  (that is, the outer layer  31 , the seat pad  28 , and the elastic installation supporting members  27 ), and thus the configuration is significantly simplified. 
     Furthermore, because the supporting rigidity of the inflator  29  is ensured due to the fixing thereof to the upper cross member  25 , and the upper cross member  25  is located in the upper portion of the seat back frame  21 , the internal space of the seat back for the airbag  30  to inflate and develop is not interfered by the inflator  29 , and thus an airbag development space for absorbing an impact at the time of a collision is ensured. 
     Moreover, the inflator  29  is disposed within the seat back frame  21  which is formed in a substantially rectangular frame shape, and thus the inflator  29  is protected against an external force by the seat back frame  21  and protrusion of the inflator  29  outwardly of the seat back frame  21  is avoided. Moreover, the inflator  29  may be integrally formed with the airbag  30  which is provided within the seat back frame  21 . 
     Eighth Implementation 
     Next, an eighth implementation will be described with reference to  FIGS. 33 and 34 . In the seventh implementation, the case has been described where an inflator is used and an airbag is caused to inflate and develop. However, in the eighth seventh implementation, the case will be described where two inflators are used and an airbag is caused to inflate and develop. 
       FIG. 33  is a schematic perspective view with partial cutaway of the front seat  10  as seen from the rear; and  FIG. 34  is a fragmentary perspective view with partial cutaway of the front seat  10 . As illustrated in  FIG. 33 , in the front seat  10  according to the present implementation, the entire configuration of the seat back  20  is the same as that of the seventh implementation except for the configuration of the inflator, and thus detailed description is omitted. 
     As illustrated in  FIGS. 33 and 34 , the inflator  29  is fixedly mounted within each of the cylindrical upper cross member  25  and the cylindrical lower cross member  26 . 
     As illustrated in  FIG. 34 , the mounting of the inflator  29  within the lower cross member  26  is achieved by having the outer circumferential surface of the cylindrical inflator  29  in contact with the inner circumferential surface of the lower cross member  26  and firmly fixing them together with a fixing bracket (not illustrated). 
     As illustrated in  FIG. 34 , the portion of the outer circumferential surface of the inflator  29  corresponding to the lower cross member  26  is provided with a plurality of small holes  26   a , and a gas suction inlet (not illustrated) is provided so that expanded gas from the inflator  29  passes through the small holes  26   a  and is introduced into the airbag  30 . 
     The mounting of the inflator  29  within the upper cross member  25  is achieved similarly to the mounting within the lower cross member  26  and description is omitted. Expanded gas jets through the small holes  25   a  opened in the upper cross member  25 . 
     According to the present implementation, in the case where another vehicle collides with a vehicle having the front seat  10 , the airbag  30  which has inflated and developed due to an operation of each inflator  29 , the outer layer  31  on the rear of the seat back  20 , the seat pad  28 , and the elastic installation supporting members  27  elastically receive rearward movement of the upper body Pb of the front seat person Pf, and thus the upper body Pb of the seated person Pf is protected. 
     On the other hand, the knees Pn of the rear seat person Pr moving forward due to a rear-end collision of a vehicle is elastically received by the outer layer  31  on the rear of the seat back  20  and the inflated and developed airbag  30 , and thus the upper body Pb of the front seat person Pf is protected against the collision impact of the knees Pn of the rear seat person Pr. Therefore, similarly to the seventh implementation, proper protection for the seated person Pf is achieved and the configuration is significantly simplified. 
     Also, because each inflator  29  is fixed within the upper cross member  25  and the lower cross member  26 , part of the bending strength and torsion strength of the upper cross member  25  and the lower cross member  26  is provided by the inflator  29 , and thus the bending rigidity and torsion rigidity of the cross members  25 ,  26  are reinforced. Therefore, as a result, the seat back frame  21  is reinforced and torsion and strain of the seat back  20  is reduced. 
     Furthermore, the inflator  29  does not protrude from the seat back frame  21 , and thus even when the inflator  29  is mounted in the seat back frame  21 , a driver does not feel uncomfortable while being seated, and inflation and development of the airbag  30  is not prevented by the inflator  29 . 
     Moreover, with this configuration, the inflators  29  disposed above and below may be operated with a time lag. By operating of the inflators  29  with a time lag, the inflation and development state of the airbag  30  may be continued longer compared with the case where the inflators  29  are operated simultaneously or the case where an inflator with a single step is operated. Thus energy absorbing effect of the airbag may last for a longer time. 
     Although the present implementation adopts a configuration in which one airbag  30  is used, two airbags may be used.  FIG. 35  is a schematic rear view with partial cutaway for explaining a modification of the present implementation; and  FIG. 36  is an operation explanatory diagram of the front seat  10  when an impact occurs according to the modification. 
     As illustrated in  FIG. 35 , in the present modification, the airbag  30  includes an upper airbag  30   a  and a lower airbag  30   b , the upper airbag being disposed on the upper side of the space between the elastic installation supporting members  27  and the outer layer  31  on the rear of the seat back  20  within the seat back  20 , the lower airbag being disposed on the lower side of the space. Each airbag is inflatably and developably retained, for example, on the outer layer  31  by a retaining unit (not illustrated). 
     The upper airbag  30   a  has upper and lower ends, the upper end being located at the height of the upper cross member  25 , the lower end being located at the height of approximately the midpoint between the upper cross member  25  and the lower cross member  26 . The upper airbag  30   a  is a bag body which is a substantially rectangular shape in a plan view and has a width with crosswise ends located close to the right and left side frames  23  and side brackets  24 , the bag body being compressed and folded in the fore-and-aft direction of the seat back  20 . 
     The lower airbag  30   b  has upper and lower ends, the lower end being located at the height of the lower cross member  26 , the upper end being located at the height of approximately the midpoint between the upper cross member  25  and the lower cross member  26 . The lower airbag  30   b  is a bag body having a width similar to the width of the upper airbag  30   a , the bag body also being compressed and folded in the fore-and-aft direction of the seat back  20 . 
     Both airbags  30   a ,  30   b  are disposed so as to be partially overlapped with each other to prevent a space therebetween when the both airbags inflate and develop. 
     With this configuration, when both upper and lower inflators  29  are operated, even when the position of the knees Pn of the rear seat person Pr is varied to some extent, the knees may be reliably received by the airbags  30   a ,  30   b  located above and below as illustrated in  FIG. 36 , and thus an impact from the rear may be received more reliably compared with the case where one airbag is used. 
     In addition, the inflation and development pattern of the entire airbag  30  is controllable by controlling on and off of the ignition of the inflators  29 . 
     Ninth Implementation 
     A ninth implementation will be described with reference to  FIGS. 37 to 39 .  FIG. 37  is a schematic explanatory cross-sectional view of the front seat  10 ;  FIG. 38  is a schematic perspective view with partial cutaway of the front seat  10  as seen from the rear; and  FIG. 39  is an operation explanatory diagram when an impact occurs. 
     The disposition and configuration of the inflator  29  according to the present implementation will be described. As illustrated in  FIG. 37 , bending portions between the upper frame  22  and the side frames  23  in the seat back frame  21  are each a corner or curved portion  21   a  which is obtained by curving or bending a tube frame. The connecting portion between each side bracket  24  and the lower cross member  26  is a corner portion  21   b  which is created when the lower cross member  26  is installed between the side brackets  24 . Also, the connecting portion between each side frame  23  and the upper cross member  25  is a corner portion  21   c  which is created when the upper cross member  25  is installed between the side frames  23 . 
     As illustrated in  FIG. 38 , two inflators  29  are mounted in the upper portion and the lower portion of the seat back frame  21 , respectively. The inflator  29  mounted in the upper portion of the seat back frame  21  has one end  101   a  and the other end  101   b , the one end  101   a  being fixed to the upper cross member  25  in the vicinity of one corner portion  21   c , the other end  101   b  being fixed to a linear portion  101   c  of a bracket  101  which is secured to the side frame  23  in the vicinity of the corner portion  21   c.    
     Similarly, the inflator  29  mounted in the lower portion of the seat back frame  21  has one end  102   a  and the other end  102   b , the one end  102   a  being fixed to the lower cross member  26  in the vicinity of one corner portion  21   b , the other end  102   b  being fixed to a linear portion  102   c  of a bracket  102  which is secured to the side bracket  24  in the vicinity of the corner portion  21   b.    
     Thus, both inflators  29  are respectively provided in the corner portions  21   c ,  21   b  which are on the diagonal of a rectangular shape including the upper cross member  25 , the side frames  23 , the side brackets  24 , and the lower cross member  26 . 
     In a vehicle having the front seat  10  according to the present implementation having the above configuration, each inflator  29  is ignited by an operation of a collision detection unit or collision information predicted by a collision prediction unit, and expanded gas from the inflator  29  thereby causes the airbag  30  to instantly inflate and develop between the elastic installation supporting members  27  and the rear surface of the seat back  20  as illustrated in  FIG. 39 . 
     In this process, the front of the airbag  30  is covered with flexible materials such as the elastic installation supporting member  27 , the seat pad  28 , and the outer layer  31 , and thus due to the inflation and development of the airbag  30 , these flexible materials exhibit slight deformation in the forward direction of the vehicle body in  FIG. 39 , whereas the outer layer  31 , which covers the rear of the airbag  30  as a flexible material, exhibits significantly protruding deformation in the rearward direction of the vehicle body. 
     In this process, the above-mentioned deformation behavior of the flexible members and flexible materials in front and in rear of the airbag  30  allows the volume E of development of the airbag  30  in the fore-and-aft direction of the vehicle body to be ensured, and also allows the energy absorbing stroke of the seat back  20  in the fore-and-aft direction of the vehicle body to be sufficiently ensured. 
     That is, as illustrated in  FIG. 32 , rearward movement, due to an impact of a rear-end collision, of the upper body Pb of the seated person Pf on the front seat  10  is elastically received by the outer layer  31  at the front of the seat back  20 , the seat pad  28 , the elastic installation supporting members  27 , and the inflated and developed airbag  30 , and thus the upper body of the seated person Pf on the front seat  10  is protected against the impact to be received when being pressed against the seat back  20  of the seat due to a rear-end collision. 
     On the other hand, due to an impact of a rear-end collision, the person Pr on the rear seat  60  is pressed against the rear seat, and is moved forward by subsequent rebound. Due to the rebound, the knees Pn of the rear seat person Pr moving forward is elastically received by the outer layer  31  on the rear of the seat back  20  and the airbag  30 , and thus the upper body Pb of the seated person Pf on the front seat  10  is protected against the collision impact of the knees Pn of the rear seat person Pr. 
     In this process, the airbag  30  is controlled by the internal geometry within the seat back  20  located immediately rearward of the upper body Pb of the seated person Pf on the front seat  10 , and is caused to stably inflate and develop. Consequently, rearward movement of the upper body Pb may be reliably received, and the impact due to forward movement of the knees Pn of the rear seat person Pr may be reliably absorbed, and thus proper protection for the seated person Pf may be achieved. 
     In addition, such protection of the seated person Pf is achieved by the airbag  30  that inflates and develops within the seat back  20  and disposition of the flexible materials in front and in rear of the airbag  30  (that is, the outer layer  31 , the seat pad  28 , and the elastic installation supporting members  27 ), and thus the configuration is significantly simplified. 
     In addition, because the inflators  29  are mounted in the corner portions  21   c  and  21   b  of the seat back frame  21  in a substantially frame shape, inflation and development of the airbag  30  is not prevented by the inflators  29 , and the torsion rigidity of the corner portions  21   c  and  21   b , which are likely to receive a stress at the time of a vehicle collision, is reinforced by the inflators  29 . 
     Furthermore, the inflators  29  are bridged between both side positions (that is, between a position of the upper cross member  25  and a position of the side frame  23 , and a position of the lower cross member  26  and a position of the side bracket  24 ) of the seat back frame  21  via the brackets  101  and  102  across the corner portions  21   c  and  21   b , and thus a reactive force to the jet of gas from each inflator  29  may be effectively distributed from both side positions of the seat back frame  21  to the entire seat back frame  21 . 
     Also, each inflator  29  is located at a corner of the rectangular shape including the upper cross member  25 , the side frames  23 , the side brackets  24 , and the lower cross member  26 , and so even when the seat back  20  is equipped with a side air bag or an active headrest mechanism, the inflators  29  may be disposed at positions that do not interfere with the side air bag or the active headrest mechanism. 
     Furthermore, with this configuration, the inflators  29  disposed in the upper portion and the lower portion of the seat back frame  21  may be operated with a time lag. By operating of the inflators  29  with a time lag, the airbag  30  may be caused to inflate and develop at a slower speed compared with the case where the inflators  29  are operated simultaneously or the case where an inflator with a single step is operated. Thus, load to a passenger due to development of the airbag may be reduced. 
     Moreover, the inflation and development state of the airbag may be continued longer compared with the case where the inflators are operated simultaneously or the case where an inflator with a single step is operated. Thus energy absorbing effect of the airbag may last for a longer time. 
     Tenth Implementation 
     Next, a tenth implementation will be described with reference to  FIGS. 40 and 41 . In the ninth implementation, the case has been described where two inflators are used and an airbag is caused to inflate and develop. However, in the tenth seventh implementation, the case will be described where four inflators are used and four airbags are caused to inflate and develop. 
       FIG. 40  is a schematic perspective view with partial cutaway of the front seat  10  as seen from the rear; and  FIG. 41  is a schematic cross-sectional view of the operation state of the front seat  10  at the time of an offset collision, the cross-sectional view corresponding to a cross-section along line XXXXI-XXXXI in  FIG. 40 . 
     As illustrated in  FIG. 40 , in the front seat  10 , the entire configuration of the seat back  20  is the same as that of the ninth implementation except for the configuration of the inflators and air bags, and thus detailed description is omitted. 
     As illustrated in  FIG. 40 , the inflators  29  are in all the respective corners (that is, the corner portions  21   c  and  21   b  at four corners) of the rectangular shape including the upper cross member  25 , the side frames  23 , the side brackets  24 , and the lower cross member  26  via brackets  103 ,  104 ,  105 , and  106 . These brackets have the same configuration as that of the brackets  101  and  102  according to the ninth implementation. 
     Four airbags  30  (hereinafter denoted as airbags  30   c ,  30   d ,  30   e , and  30   f ) are disposed which are obtained by dividing a rectangular shape into four equal rectangular-shaped parts by the midpoints of the frames and the cross members in a rear view, the rectangular shape including the upper cross member  25 , the side frames  23 , the side brackets  24 , and the lower cross member  26  within the substantially rectangular frame-shaped seat back frame  21 . Each airbag is inflatably and developably retained, for example, on the outer layer  31  by a retaining unit (not illustrated). 
     The airbags  30   c ,  30   d ,  30   e , and  30   f  are connected to the respective inflators  29  located at four corners of the above-mentioned rectangular shape in the seat back frame  21  via respective gas suction inlets (not illustrated), and are disposed so as to be partially overlapped with each other to some extent to prevent a space between airbags  30  when all the airbags  30  inflate and develop. 
     The front seat  10  allows, for example, the above-mentioned collision detection unit (not illustrated) or collision prediction unit to detect collision information, and according to the information, causes a control unit (not illustrated) to control a inflator  29  to be ignited, and appropriate airbag(s) out of four airbags are selectively caused to inflate and develop at the time of a collision. If some of the airbags  30  have a higher load, the load may be reliably absorbed. This is effective for example when an offset collision occurs. 
     As a specific example, the case will be described where a vehicle experiences a rear-end collision from the rear left. First, when a vehicle experiences a rear-end collision from the rear left, the vehicle exhibits yawing to the forward right, and the upper body Pb of the seated person Pf on the front seat  10  is pressed against the seat back  20  to the rear left. Similarly, the upper body of the seated person Pr on the rear seat is pressed against to the rear left, then the knees are thrown to the forward left by subsequent rebound. 
     In this process, the collision detection unit (not illustrated) or the collision prediction unit recognizes a rear-end collision from the rear left, and the inflators  29  to be ignited are limited to the two inflators  29  located inwardly along the vehicle width in the seat back  20  by a control unit (not illustrated) based on recognized collision information. As illustrated in  FIG. 41 , two airbags  30   c  and  30   d  are caused to inflate and develop, thereby making it possible to reliably receive movement of the upper body Pb of the seated person Pf to the rear left (that is, in the direction of arrow A) and movement of the knees Pn of the seated person Pr to the forward left (that is, in the direction of arrow B). 
     Therefore, with the front seat  10  according to the present implementation, it is possible to individually control multiple airbags  30  in conjunction with a collision, and only necessary airbags  30  are selectively caused to inflate and develop according to the type of a collision as described above, and thus an impact may be absorbed accurately. 
     Furthermore, a truss structure may be constructed in which each of the inflators  29  is bridged over a corresponding one of the four corners of the rectangular shape of the seat back frame  21 , and thus the inflation and development of each airbag  30  is not prevented by the structure and the rigidity of the seat back frame  21  is reinforced. 
     The four airbags  30  disposed within a substantially rectangular shape of the seat back frame  21  are each caused to inflate and develop by an individual inflator  29 , and thus a larger reactive force of the airbag may be ensured compared with the case where the volume corresponding to the combined volume of the four airbags  30   c  to  30   f  is caused to inflate and develop with an inflator  29 . 
     It is to be noted that the present disclosure is not limited to the above-described implementations, and various modifications may be made without departing from the scope of the present disclosure. For example, the first implementation adopts a configuration in which the inflator  30  is mounted on the side bracket  24  outwardly in the vehicle body width. However, it is also possible to mount the inflators  30  both on the side bracket  24  outwardly in the vehicle body width and on the side bracket  24  inwardly in the vehicle body width. With this configuration, operating the inflators  29  with a time lag makes it possible to change the duration of inflation and development and the amount of inflation and development of each airbag. 
     Furthermore, in each implementation, the vehicle has the front seat  10  and the rear seat  60  that are arranged side by side. However, the rear seat  60  is not a required component. For example, the rear of the front seat  10  may serve as a loading bed. Also in this case, it is possible to protect a seated person on the vehicle seat  10  against an impact by the inflation and development of the airbag  30 , the impact being generated by a loaded object on the loading bed, which is moved forward due to a vehicle collision to collide with the rear of the seat back  30 . 
     In the eighth implementation, a configuration has been illustrated in which the inflator  29  is mounted to the upper cross member  25 . However, the inflator  29  may be disposed at a location outwardly in the vehicle width direction of the positions in the upper frame  22 , at which the cylindrical stay brackets  42  are formed, the location being outward of the seat back frame  21 . With this configuration, interference between the airbag  30  which inflates and develops and each inflator is completely avoidable, and thus stable development of the airbag  30  is ensured. 
     When an inflator is mounted in the upper portion of the seat back frame  21 , it is also optional to use an inflator which causes an airbag for active headrest to inflate and develop. 
     In the ninth implementation, the inflator  29  is mounted in the corner portions  21   c  or  21   b . However, without being limited to these portions, the inflator  29  may be mounted to one of the corner or curved portions  21   a  between the upper frame  22  and the side frames  23 , the corner or curved portions  21   a  being obtained by curving or bending a tube frame.