Patent Publication Number: US-8123292-B2

Title: Vehicle seat

Description:
TECHNICAL FIELD 
     The present invention relates to a vehicle seat. More particularly, the present invention relates to a vehicle seat having a headrest moving mechanism that is capable of moving a support portion of a headrest for supporting a head of a sitting person with respect to a seat back when a vehicle back-side collision is detected, thereby placing the support portion closer to the head. 
     BACKGROUND ART 
     Conventionally, there is a vehicle seat having a mechanism that is capable of instantaneously moving a headrest forwardly so as to support a head of a sitting person when a vehicle back-side collision happens. For example, Japanese Laid-Open Patent Publication No. 2002-142910 teaches a technique in which a support portion for supporting a head is moved forwardly from a headrest base portion using a four-joints link mechanism. 
     In this disclosure, the four-joints link mechanism is constructed of two parallel links that are disposed between the headrest base portion and the support portion. The two parallel links are retained in a postural condition in which the support portion is retracted toward the headrest base portion when the vehicle back-side collision does not yet happen. When the vehicle back-side collision happens, the condition is released. Upon release of the condition, the two parallel links is rotated so as to move the support portion forwardly by a biasing force. 
     DISCLOSURE OF THE INVENTION 
     Problem to be Solved by the Invention 
     However, in the conventional technique taught by the above-mentioned document, a mechanism for moving the support portion forwardly needs a plurality of linking members and connecting members. This may lead to increased number of parts and increased weight. 
     The present invention has been made in order to solve the above-mentioned problem. It is an object of the present invention to simplify a mechanism that is capable of moving a support portion of a headrest for supporting a head of a sitting person when a vehicle back-side collision happens, thereby preventing the head from being inclined rearwardly. 
     Means for Solving the Problem 
     In order to solve the above-mentioned problem, a vehicle seat of the present invention uses a following means. 
     First, a first invention is a vehicle seat having a headrest moving mechanism that is capable of relatively moving a support portion of a headrest for supporting a head of a sitting person with respect to a seat back when a vehicle back-side collision is detected, so as to position the support portion closer to the head. The headrest moving mechanism includes a linking member that links the support portion of the headrest and a headrest base portion supported on the seat back to each other, and a guide pathway formed in the headrest base portion. The guide pathway is capable of slidably receiving the support portion in order to determine a postural position of the support portion of which the posture with respect to the headrest base portion can be rotationally displaced via the linking member. The guide pathway is shaped so as to move the support portion forwardly and upwardly with respect to the headrest base portion from an initial position in a time before the vehicle back-side collision happens to a collision preparatory position as the linking member is rotated. 
     According to the first invention, the support portion of the headrest for supporting the head of the sitting person slidably engages the guide pathway, so as to be normally retained in a posture of the initial position with respect to the headrest base portion. When the vehicle back-side collision is detected, as the linking member is rotated, the support portion moves forwardly and upwardly from the initial position to the collision preparatory position while sliding along the guide pathway. 
     Next, in a second invention related to the first invention, the headrest moving mechanism includes a stopper mechanism, so that in a condition in which the support portion reaches the collision preparatory position and in which an engagement portion provided to the support portion is restrained from moving in an advancing direction within the guide pathway, a loading caused by rearward inclination of the head and applied to the support portion can function as a pressing force that urges the engagement portion of the support portion to further move in the advancing direction. 
     According to the second invention, when the support portion reaches the collision preparatory position, the engagement portion provided to the support portion is restrained from sliding in the advancing direction with respect to the guide pathway. In this condition, when the loading caused by rearward inclination of the head is applied to the support portion, the loading urges the engagement portion to further slide in the advancing direction within the guide pathway. Therefore, the support portion cannot be pushed back even if the support portion is applied with the loading from the head, the support portion can catch the head in the collision preparatory position. 
     Next, in a third invention related to the first or second invention, the guide pathway formed in the headrest base portion is formed so as to have a concave shape depressed in a width direction of the headrest base portion. The engagement portion provided to the support portion is formed so as to have a convex shape that is capable of engaging the concaved guide pathway. A depressed stopper groove is formed in the guide pathway that guides the engagement portion from the initial position to the collision preparatory position. The stopper groove is capable of receiving the engagement portion when the engagement portion is pushed rearwardly within the guide pathway. The stopper groove is constructed to receive the engagement portion and to prevent the engagement portion from being pushed back rearwardly when the engagement portion is pressed by a loading from the head while the engagement portion is moving in the advancing direction. 
     According to the third invention, when the vehicle back-side collision is detected, if the support portion is applied with the loading from the head while the support portion is moving in the advancing direction toward the collision preparatory position, the engagement portion is applied with a corresponding force and is pushed back so as to enter the stopper groove. As a result, the support portion is in a position in which the support portion is restrained from further being pushed back. Therefore, the support portion can catch the head in this position. 
     Further, in a fourth invention related to the third invention described above, an engagement prevention mechanism is provided to the headrest base portion. When the engagement portion is moving in a retracting direction from the collision preparatory position toward the initial position, the engagement prevention mechanism is capable of prohibiting the engagement portion from entering the stopper groove formed in the guide pathway, thereby allowing the engagement portion to pass over the stopper groove. When the engagement portion moves within the guide pathway in the advancing direction from the initial position toward the collision preparatory position, the engagement prevention mechanism allows the engagement portion to enter the stopper groove on the way in an advancing movement thereof. 
     According to the fourth invention, if the support portion is applied with the loading from the head while the support portion moving toward the collision preparatory position, the engagement portion enters the stopper groove on the way in the advancing movement thereof. However, even if the support portion is applied with the loading from the head when the engagement portion is moving from the collision preparatory position toward the initial position, the engagement portion can be prohibiting from entering the stopper groove by the engagement prevention mechanism, so that the engagement portion can pass over the stopper groove. 
     Next, in a fifth invention related to the fourth invention described above, the engagement prevention mechanism comprises an arm-shaped prevention lever and a spring member biasing the prevention lever. The prevention lever is rotatably connected to the headrest base portion. The spring member maintains the prevention lever in a postural condition in which the prevention lever is exposed to the guide pathway. The prevention lever has a receiving portion that is formed in a forward end portion thereof and is capable of catching the engagement portion moving in the retracting direction. The prevention lever is capable of rotating and guiding the engagement portion caught by the receiving portion so as to move the same beyond the stopper groove. When the engagement portion moves in the advancing direction, the prevention lever allows the engagement portion to move in the advancing direction while pushed away by the engagement portion. When the engagement portion moves closer to the collision preparatory position, the prevention lever is disengaged from the engagement portion, so as to be returned to the postural condition in which the prevention lever is capable of catching the engagement portion moving in the retracting direction. 
     According to the fifth invention, the prevention lever is normally maintained by the spring member in the postural condition in which the prevention lever is exposed to the guide pathway. When the support portion moves in the advancing direction, the prevention lever allows the engagement portion to enter the stopper groove while pushed away by the engagement portion sliding in the advancing direction. However, when the support portion moves in the retracting direction from the collision preparatory position, the prevention lever is capable of catching the sliding engagement portion in the receiving portion thereof, thereby moving the engagement portion beyond the stopper groove while rotating. 
     Next, in a sixth invention related to any of the third to fifth inventions described above, a plurality of stopper grooves are formed in the guide pathway so as to be positioned therealong. 
     According to the sixth invention, a plurality of stopper grooves are formed in the guide pathway so as to be positioned therealong. Therefore, when the support portion is applied with the loading from the head while the support portion is moving in the advancing direction, the engagement portion can enter one of the stopper grooves that is positioned closest thereto. 
     Next, in a seventh invention related to any of the first to fifth inventions described above, the guide pathway extends forwardly and upwardly with respect to the headrest base portion. 
     According to the seventh invention, the guide pathway extends forwardly and upwardly so as to correspond to a moving direction of the support portion. Therefore, the support portion can move while substantially maintaining an inclined posture thereof with respect to the headrest base portion. 
     Effect of the Invention 
     The means described above may provide following effects. 
     According to the first invention, due to simple construction in which the support portion is linked to the headrest base portion and slidably engages the guide pathways, the support portion can move from the initial position to the collision preparatory position that can prevent the rearward inclination of the head while maintaining an inclined posture of the support portion with respect to the headrest base portion. 
     Further, according to the second invention, the stopper mechanism is provided for preventing the support portion from being pushed back even if the support portion is applied with a loading from the head when the support portion reaches the collision preparatory position, and the stopper mechanism is constructed of an engagement structure in which the support portion and the headrest base portion are slidably engaged with each other. Therefore, construction for preventing the rearward inclination of the head can be simplified and streamlined. 
     Further, according to the third invention, the stopper groove that is capable of preventing the support portion from being pushed back by the head is formed in the guide pathway along which the support portion slides. Therefore, even when the support portion is moving in an advancing direction, the head can be prevented from being inclined rearwardly. As a result, such construction can be simplified. 
     Further, according to the fourth invention, an engagement prevention mechanism is provided. When the support portion is moving in the advancing direction, the engagement prevention mechanism allows the stopper portion to engage the stopper groove on the way in an advancing movement thereof. Conversely, when the support portion is moving in a retracting direction from the collision preparatory position, the engagement prevention mechanism is capable of guiding the support portion such that the support portion can pass over the stopper groove. Therefore, the support portion can be smoothly returned to the initial position. 
     Further, according to the fifth invention, the engagement prevention mechanism is constructed of an arm-shaped prevention lever and a spring member retaining the prevention lever in an initial position. Therefore, the engagement prevention mechanism can be constructed of relatively simple components. 
     Further, according to the sixth invention, a plurality of stopper grooves are formed in the guide pathway so as to be positioned therealong. Therefore, when the support portion is moving in the advancing direction, the support portion can be prevented from being pushed back by the head in a plurality positions on the way in the advancing movement thereof. 
     Further, according to the seventh invention, the guide pathway extends forwardly and upwardly so as to correspond to a moving direction of the support portion. Therefore, the support portion can move while substantially maintaining an inclined posture thereof. As a result, a supporting posture of the support portion for supporting the head can be maintained. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a vehicle seat according to Embodiment 1, which schematically illustrates construction thereof. 
         FIG. 2  is a structural diagram, which illustrates a structure in which a push rod is pushed upwardly by an operation cable. 
         FIG. 3  is an enlarged perspective view, which illustrates an insertion structure in which a stay of a headrest and the operation cable are inserted into a cylindrical support. 
         FIG. 4  is an enlarged perspective view of a detection device that can detect a vehicle back-side collision. 
         FIG. 5  is a side view, which illustrates a condition in which a support portion of the headrest is retained in an initial position. 
         FIG. 6  is a schematic view of internal structure of a headrest moving mechanism, which corresponds to a sectional view taken along line VI-VI of  FIG. 11 . 
         FIG. 7  is a side view, which illustrates a condition in which the support portion of the headrest is moving toward a head. 
         FIG. 8  is a side view, which illustrates a condition in which the support portion of the headrest reaches a collision preparatory position. 
         FIG. 9  is a side view, which illustrates a condition in which the support portion of the headrest is moving from the collision preparatory position toward the initial position. 
         FIG. 10  is a side view, which illustrates a condition in which the support portion of the headrest is moved toward the initial position while it is guided. 
         FIG. 11  is a structural diagram of the headrest moving mechanism, which is viewed from line XI of  FIG. 8 . 
         FIG. 12  is a perspective view, which illustrates a condition before the stay is inserted into the cylindrical support. 
         FIG. 13  is a perspective view, which illustrates a condition in which the stay is inserted into the cylindrical support. 
         FIG. 14  is a sectional view taken along line XIV-XIV of  FIG. 13 . 
         FIG. 15  is a schematic view of a modified form of a headrest moving mechanism. 
         FIG. 16  is a schematic view of another modified form of a headrest moving mechanism. 
     
    
    
     DESCRIPTION OF SYMBOLS 
       1  vehicle seat 
       2  seat back 
       2 F back frame 
     Fu upper frame 
     Fs side frame 
       2 S support 
     Sb knob 
     St engagement claw 
     Sd insertion groove 
       3  seat cushion 
       4  headrest 
       4 A support portion 
       4 B stay 
     Bk engagement groove 
     Bd reception groove 
       4 C headrest base portion 
       10  headrest moving mechanism 
       11 B rear surface portion 
       11 D bottom surface portion 
       11 S side surface portion 
       11 U top surface portion 
       11 R rib 
       11 H elongated hole (guide pathway) 
     H 0  lower end portion 
     H 1  first stopper groove (stopper groove) 
     H 2  second stopper groove (stopper groove) 
     H 3  upper end portion 
       12  linking member 
       12 A connection shaft 
       12 B connection shaft 
       13  support member 
       13 A connection shaft (engagement portion) 
       14  hook 
       14 A connection shaft 
       14 B upper jaw portion 
       14 C lower jaw portion 
       14 D engagement groove 
       14 S torsion spring 
       15  engagement-disengagement member 
       15 A operation arm portion 
       15 B connection shaft 
       15 C engagement arm portion 
       15 S torsion spring 
       16  tension spring 
       17  prevention lever 
       17 A connection shaft 
       17 B receiving portion 
       17 S torsion spring (spring member) 
       20  pressure receiving member 
       20 S torsion spring 
       21  pressure receiving portion 
       30  damper 
       31  rotation shaft 
       31 A connection arm 
       32  case 
       32 A operation arm 
       40  operation cable 
       41  inner member 
       41 P engagement projection 
       42  outer member 
       42 S elongated hole 
       42 H head portion 
       42 D bulged portion 
       50  push rod 
       60  attachment bracket 
       61  outer attaching portion 
       62  stopper 
     BEST MODE FOR CARRYING OUT THE INVENTION 
     In the following, the best mode for carrying out the present invention is described with reference to the drawings. 
     Embodiment 1 
     First, construction of a vehicle seat according to Embodiment 1 is described with reference to  FIGS. 1 to 14 . 
       FIG. 1  is a perspective view of the vehicle seat  1 , which schematically illustrates construction thereof. The vehicle seat  1  is composed of a seat back  2  that functions as a back support of a sitting person, a seat cushion  3  that functions as a seating portion, and a headrest  4  that functions as a head support. In the drawings such as  FIG. 1 , in order to clarify inner structure of the seat back  2 , the headrest  4  and other components, covering structure thereof is omitted. 
     The headrest  4  has two rod-shaped stays  4 B and  4 B that are vertically attached to a lower portion thereof. The stays  4 B and  4 B are respectively inserted into insertion ports Sa formed in cylindrical supports  25  and  2 S that are attached to an upper surface portion of the seat back  2 , so that the headrest  4  is attached to the upper surface portion of the seat back  2 . The supports  25  and  2 S are integrally secured to an upper frame Fu of a back frame  2 F that constitutes a framework of the seat back  2 . The upper frame Fu is integrally connected to both of side frames Fs and Fs, so as to connect upper end portions of the side frames Fs and Fs. 
     The headrest  4  is normally retained in its predetermined position, so as to catch the head of the sitting person at a rear side position thereof. However, the headrest  4  is constructed such that when a vehicle back-side collision happens, a support portion  4 A can instantaneously move forwardly, so as to move closer to the head. The support portion  4 A is positioned at a front side of the headrest  4  and is constructed to catch the head. That is, the headrest  4  is constructed such that when the vehicle back-side collision, only the support portion  4 A of the headrest  4  can move to a position immediately behind the back of the head of the sitting person that has a posture in which the body is forwardly spaced from the seat back  2  and the headrest  4 . Thus, when the vehicle back-side collision happens, rearward inclination of the head can be quickly prevented by the support portion  4 A. As a result, a loading applied to the neck can be reduced, so that a whiplash injury can be prevented. 
     Motion to move the support portion  4 A forwardly when the vehicle back-side collision happens can be performed by a headrest moving mechanism  10  that is incorporated into the headrest  4 . As shown in  FIG. 5 , in a normal condition in which the vehicle back-side collision does not yet happen, the headrest moving mechanism  10  retains the support portion  4 A in a posture of an initial position thereof while maintaining the support portion  4 A in a forward movement restraint condition. Further, the support portion  4 A is normally biased in a forward moving direction, i.e., in a direction toward the head, by a tension spring  16  that is positioned between the support portion  4 A and a headrest base portion  4 C integrated with the stays  4 B and  4 B. Therefore, in the normal condition in which the vehicle back-side collision does not yet happen, the support portion  4 A is retained in the initial position against a biasing force of the tension spring  16 . 
     When the vehicle back-side collision happens and the movement restraint condition of the support portion  4 A is canceled, the headrest moving mechanism  10  can move the support portion  4 A forwardly by the biasing force of the tension spring  16 . At this time, the headrest moving mechanism  10  can move the support portion  4 A forwardly and upwardly along profiles of elongated holes  11 H and  11 H formed in the headrest base portion  4 C which will be described hereinafter. As a result, as shown in  FIG. 8 , the support portion  4 A is moved to the position immediately behind the back of the head (a collision preparatory position). The headrest moving mechanism  10  is constructed such that in the condition in which the support portion  4 A is moved to the collision preparatory position, the support portion  4 A cannot be pushed back even if the support portion  4 A is applied with a loading caused by the rearward inclination of the head when the vehicle back-side collision happens. As a result, the head of the sitting person can be stably caught by the support portion  4 A in the collision preparatory position. 
     Further, the elongated holes  11 H and  11 H correspond to guide pathways in the present invention. 
     Referring to  FIG. 1  again, an operation of canceling the movement restraint condition of the support portion  4 A in the initial position as described above can be performed by a push up motion of a push rod  50 . The push rod  50  is inserted into the right side tubular stay  4 B of the headrest  4 , as seen in the drawing. 
     As shown in  FIG. 2 , an upper end portion of the push rod  50  is connected to an engagement-disengagement member  15  that is provided as an operation member of the headrest moving mechanism  10 . Further, a lower end portion of the push rod  50  is connected to an upper end portion of an operation cable  40  that is disposed inside the seat back  2 . 
     As shown in  FIG. 4 , a lower end portion of the operation cable  40  is connected to a pressure receiving member  20  that is disposed in the seat back  2 , so that the operation cable  40  can be pulled downwardly when the vehicle back-side collision happens. The pressure receiving member  20  may function as a detection device of the vehicle back-side collision. Further, as shown in  FIG. 2 , the operation cable  40  is constructed such that when its lower end portion is pulled, a corresponding operational force is transmitted to its upper end portion in reverse, thereby pushing up the push rod  50 . 
     Further, when the push rod  50  is pushed up, the engagement-disengagement member  15  is pushed and rotated counterclockwise as seen in the drawing, so that the movement restraint condition of the support portion  4 A can be canceled. 
     In the following, construction of the above-mentioned detection device of the vehicle back-side collision, i.e., the detection device for pushing up the push rod  50  when the vehicle back-side collision happens, is described. 
     As shown in  FIG. 4 , the bent rod-shaped pressure receiving member  20  is disposed in the seat back  2  so as to be positioned in a middle portion thereof. The pressure receiving member  20  is positioned so as to extend in a width direction. A right end portion of the pressure receiving member  20  as seen in the drawing is rotatably supported on a right side frame Fs of the seat back  2 . Further, a left end portion of the pressure receiving member  20  as seen in the drawing is rotatably supported on a left side frame Fs via a rotary type damper  30 . 
     Formed in a widthwise middle portion of the pressure receiving member  20  is a pressure receiving portion  21  that capable of receiving a seat back loading applied by the sitting person. The pressure receiving portion  21  is formed by bending the right end portion of the pressure receiving member  20  downwardly as seen in the drawing, so as to be offset from a rotation center thereof. Thus, when the pressure receiving portion  21  is pressed rearwardly by the seat back loading applied by the sitting person, the pressure receiving member  20  can be rotated about the both end portions thereof. 
     Further, a torsion spring  20 S is disposed between the right end portion of the pressure receiving member  20  and the side frame Fs. The torsion spring  20 S is disposed while it is previously twisted, thereby rotationally biasing the pressure receiving member  20  in such a direction as to push the pressure receiving portion  21  forwardly. As a result, the pressure receiving member  20  is normally maintained in a postural condition in which the pressure receiving portion  21  is pressed against a cushion pad (not shown) that is attached to a seat back surface of the seat back  2 . 
     Further, a well-known rotary type damper is used as the damper  30  that is connected to the left end portion of the pressure receiving member  20 . That is, in the damper  30 , a rotation shaft  31  is inserted into a cylindrical case  32 . The rotation shaft  31  and the case  32  are assembled so as to be rotated relative to each other. 
     The rotation shaft  31  has a connection arm  31 A that is attached to a right end portion thereof as seen in the drawing. The connection arm  31 A is integrally connected to the left end portion of the pressure receiving member  20 . Further, a left end portion of the rotation shaft  31  as seen in the drawing is rotatably supported by the left side frame Fs. Thus, the case  32  is supported by the rotation shaft  31 , so as to be rotatable with respect to the side frame Fs. Further, the case  32  has an operation arm  32 A that is attached to an outer circumferential surface thereof. The operation arm  32 A is connected to a lower end portion of an inner member  41  of the operation cable  40  which will be described hereinafter. The case  32  is constructed such that when the operation arm  32 A contacts a stopper  62  of an attachment bracket  60  that is attached to the side frame Fs, its movement in such a direction as to contact the stopper  62  can be restrained. 
     The case  32  described above is filled with a viscous fluid such as silicone oil and is hermetically sealed. As a result, when the rotation shaft  31  is urged to rotate relative to the case  32 , a viscous resistance is produced therebetween dependent upon a rotating speed thereof. This viscous resistance is applied between the rotation shaft  31  and the case  32 . The viscous resistance is increased as the rotating speed of the rotation shaft  31  is increased. Conversely, the viscous resistance is decreased as the rotating speed of the rotation shaft  31  is decreased. When the applied viscous resistance is large, a rotational force of the rotation shaft  31  can be easily transmitted to the case  32 . Conversely, when the applied viscous resistance is small, the rotational force of the rotation shaft  31  cannot be not easily transmitted to the case  32 . 
     When the sitting person reclines against the seat back  2 , the pressure receiving member  20  and the damper  30  thus constructed may operate as follows. 
     First, in the normal condition in which the vehicle back-side collision does not yet happen, when the sitting person reclines against the seat back  2 , the pressure receiving member  20  is pushed and rotated rearwardly at a relatively gentle speed corresponding to behavior of the sitting person. Therefore, in this case, the rotation shaft  31  can rotate relative to the case  32  at a relatively gentle speed, so that the applied viscous resistance is small. As a result, the rotation shaft  31  idles within the case  32 , so that the rotational force of the rotation shaft  31  can not be transmitted to the case  32 . 
     However, when the vehicle back-side collision happens, the sitting person is sharply pressed against the seat back  2  by impact of the collision. At this time, the pressure receiving member  20  is pushed and rotated rearwardly at a relatively high speed corresponding to impulsive motion of the setting person. Therefore, in this case, the rotation shaft  31  can relatively rotate at a relatively high speed, so that the applied viscous resistance is large. As a result, the rotational force of the rotation shaft  31  can be transmitted to the case  32 , so that the case  32  can rotate integrally with the rotation shaft  31 . Thus, the case  32  can pull the lower end portion of the inner member  41  of the operation cable  40  downwardly because the lower end portion of the inner member  41  is connected to the operation arm  32 A. 
     Further, when the lower end portion of the inner member  41  of the operation cable  40  is pulled, as shown in  FIG. 2 , the corresponding operational force is transmitted to an upper end portion of the inner member  41 , so that the push rod  50  is pushed up within the support  2 S. 
     Next, a transmission mechanism of the operational force transmitted from the operation cable  40  to the push rod  50  is described. 
     The operation cable  40  has a double layer structure in which the linear inner member  41  is inserted into a flexible tubular outer member  42 . The inner member  41  has flexibility greater than the outer member  42 . As shown in  FIG. 3 , the operation cable  40  is disposed inside the seat back  2 , and the lower end portion of the inner member  41  is connected to the operation arm  32 A of the damper  30  as described above. Further, a lower end portion of the outer member  42  is connected to an outer attaching portion  61  of the attachment bracket  60  that is attached to the left side frame Fs as seen in the drawing. Thus, the operation cable  40  is constructed such that the lower end portion of the inner member  41  is pulled from the lower end portion of the outer member  42  when the vehicle back-side collision happens. 
     As shown in  FIGS. 2 and 3 , upon insertion of the upper end potion of the operation cable  40  into the support  2 S from below, the operation cable  40  can be assembled to the support  2 S so as to push the push rod  50  disposed in the cylindrical stay  4 B that is inserted into the cylindrical support  2 S. 
     In particular, as shown in  FIG. 3 , the operation cable  40  is constructed such that T-shaped engagement projections  41 P and  41 P formed in the upper end portion of the inner member  41  are projected radially outwardly from elongated through holes  425  and  42 S that are formed in a circumferential wall of the upper end of the outer member  42 . 
     As a result, the inner member  41  can axially move relative to the outer member  42  within a range corresponding to a range that the T-shaped engagement projections  41 P and  41 P can move within the elongated holes  425  and  42 S. The engagement projections  41 P and  41 P and the elongated holes  425  and  42 S are axisymmetrically formed in two circumferential positions of the inner member  41  and the outer member  42 . A head portion  42 H is formed in an upper end portion of the outer member  42 , so as to close a tubular end portion thereof. 
     As shown in  FIG. 12 , when the upper end portion of the operation cable  40  thus constructed is inserted into the cylindrical the support  2 S from below, the operation cable  40  is temporarily retained while the upper end portion thereof is suspended from the support  2 S. In this suspended condition, as shown in  FIG. 13 , upon insertion of the stay  4 B into the cylindrical support  2 S from above, the operation cable  40  can be transferred from a condition in which the operation cable  40  is suspended from the support  2 S to a condition in which the operation cable  40  is suspended from the stay  4 B. 
     Further, when the operation cable  40  is in the condition in which it is suspended from the stay  4 B, the operation cable  40  is in a condition in which the operation cable  40  can transmit the operational force produced from the lower end thereof being pulled to the push rod  50  positioned inside the stay  4 B in reverse as a pushing operational force. 
     The construction described above is described in detail with reference to  FIG. 3 . First, formed in a circumferential wall of the support  2 S are slot-shaped insertion grooves Sd and Sd that extend axially upwardly from a lower end portion of the support  2 S. The insertion grooves Sd and Sd are positioned in two circumferential positions of the support  2 S so as to be axisymmetrical with each other. The insertion grooves Sd and Sd are respectively shaped such that the engagement projections  41 P and  41 P formed in the inner member  41  of the operation cable  40  can be received therein and can be inserted thereinto in an axial direction. 
     The insertion grooves Sd and Sd are respectively shaped such that terminal end portions corresponding to upper end sides thereof that receive the engagement projections  41 P and  41 P are respectively circumferentially curved leftwardly and rightwardly as seen in the drawings. In particular, the insertion groove Sd positioned in a near side in  FIG. 3  and shown by solid lines is shaped such that the terminal end portion thereof is gently curved leftwardly as seen in the drawing. Conversely, the insertion groove Sd positioned in a far side in  FIG. 3  and shown by broken lines is shaped such that the terminal end portion thereof is curved rightwardly as seen in the drawing. That is, the insertion groove Sd positioned in the far side in  FIG. 3  is shaped so as to be axisymmetrical with the insertion groove Sd positioned in the near side in  FIG. 3 . 
     Each of the circumferentially curved terminal end portions of the insertion grooves Sd and Sd is shaped so as to be positioned below a horizontal level line. As a result, when the engagement projections  41 P and  41 P (the operation cable  40 ) are inserted into the insertion grooves Sd and Sd until they reach terminal end positions thereof, the engagement projections  41 P and  41 P are stably retained while suspended from the support  2 S, so as to be prevented from falling therefrom under their own weight. 
     The operation cable  40  can be inserted into the cylindrical support  2 S by simply inserting the same upwardly (axially) after the engagement projections  41 P and  41 P attached to the upper end portion of the inner member  41  are inserted into the insertion grooves Sd and Sd. Further, when the engagement projections  41 P and  41 P reach the axial terminal end positions of the insertion grooves Sd and Sd, the operation cable  40  is circumferentially rotated along to the curved terminal end portions. Thus, as shown in  FIG. 12 , the engagement projections  41 P and  41 P reach the circumferentially curved terminal end positions of the insertion grooves Sd and Sd, so that the operation cable  40  can be retained while suspended from the support  2 S. 
     Referring to  FIG. 3  again, the upper end portion of the outer member  42  is integrally formed from a synthetic resin, so as to have a bulged portion  42 D that is partially bulged radially outwardly. The bulged portion  42 D is positioned at an axial mid point of the upper end portion of the outer member  42  that is inserted into the cylindrical support  2 S. The bulged portion  42 D is formed over the entire circumference thereof and has a serration shape. The bulged portion  42 D is formed to have an outer diameter substantially equal to an inner diameter of the cylindrical support  2 S. Therefore, because the bulged portion  42 D can be gently fitted into the cylindrical support  2 S when the upper end portion of the operation cable  40  is inserted into the cylindrical support  2 S, the operation cable  40  can be smoothly inserted without swinging within the cylindrical support  2 S. 
     Further, as shown in  FIG. 12 , slot-shaped reception grooves Bd and Bd are formed in a circumferential wall of the stay  4 B that is inserted into the cylindrical support  2 S from above. The reception grooves Bd and Bd are formed so as to extend axially upwardly from an lower end portion of the stay  4 B. The reception grooves Bd and Bd are positioned in two circumferential positions of the stay  4 B so as to be axisymmetrical with each other. When the stay  4 B is inserted into the cylindrical support  2 S from above, the reception grooves Bd and Bd may receive the engagement projections  41 P and  41 P of the inner member  41  of the operation cable  40  that is suspended in the cylindrical support  2 S. Further, when the stay  4 B is further inserted into the support  2 S after the engagement projections  41 P and  41 P are received in the reception grooves Bd and Bd, the engagement projections  41 P and  41 P are introduced upwardly (axially) along the reception grooves Bd and Bd. 
     The reception grooves Bd and Bd that receive the engagement projections  41 P and  41 P are respectively shaped such that terminal end portions corresponding to upper end sides thereof are circumferentially curved in opposite directions to the insertion grooves Sd and Sd formed in the support  2 S described above. Each of the curved terminal end portions of the reception grooves Bd and Bd is shaped so as to be gently curved from an axial direction to a horizontal direction. As a result, when the stay  4 B is inserted into the cylindrical support  2 S, the engagement projections  41 P and  41 P positioned in the cylindrical support  2 S can be smoothly received in the reception grooves Bd and Bd by a push-in operational force axially applied to the stay  4 B until they reach the terminal end positions of the reception grooves Bd and Bd that are directed horizontally. 
     Therefore, as shown in  FIG. 13 , when the stay  4 B is inserted into the cylindrical support  2 S from above, the engagement projections  41 P and  41 P are circumferentially rotated while guided by the curved portion of the reception grooves Bd and Bd formed in the stay  4 B. As a result, the engagement projections  41 P and  41 P are pushed back from a condition in which the engagement projections  41 P and  41 P are retained in the terminal end positions of the insertion grooves Sd and Sd and are moved in such a direction as to be released from this condition. When the engagement projections  41 P and  41 P move along the curved portions of the reception grooves Bd and Bd and reach the terminal end positions thereof, the engagement projections  41 P and  41 P are pushed back to axially extended portions of the insertion grooves Sd and Sd. 
     Thus, the engagement projections  41 P and  41 P are transferred from the condition in which they are suspended from the support  2 S such that axial movement thereof is restrained to the condition in which they are suspended from the stay  4 B. As a result, the engagement projections  41 P and  41 P are placed in a condition in which their axial movement with respect to the stay  4 B is restrained. That is, the engagement projections  41 P and  41 P are released from the axial movement restraint condition in which their axial movement with respect to the support  2 S is restrained, so as to be placed in the condition in which their axial movement with respect to the stay  4 B is restrained. 
     At this time, because the engagement projections  41 P and  41 P are positioned in the axially extended portions of the insertion grooves Sd and Sd, the engagement projections  41 P and  41 P are placed in a condition in which their circumferential movement with respect to the support  2 S is restrained. As a result, the engagement projections  41 P and  41 P are retained in the terminal end positions of the reception grooves Bd and Bd while guided by the axially extended insertion grooves Sd and Sd. Thus, the inner member  41  of the operation cable  40  and the stay  4 B are axially integrally connected to each other, so as to be placed in a condition in which they can axially move together with each other with respect to the support  2 S. 
     As shown in  FIG. 14 , disposed in the cylindrical support  2 S is a plate-shaped engagement claw St that is capable of engaging recess-shaped engagement grooves Bk that are formed in the outer circumferential wall of the stay  4 B. The engagement claw St is normally applied with a biasing force and is maintained in a postural condition in which the engagement claw St is projected into the cylindrical support  2 S. The engagement claw St can be retracted from the cylindrical support  2 S by pushing a knob Sb from the side. 
     Thus, when a insertion position of one of the engagement grooves Bk is aligned with the engagement claw St upon insertion of the stay  4 B into the cylindrical support  2 S, the engagement claw St enters and engages the corresponding engagement groove Bk by the biasing force. As a result, the stay  4 B is placed in a condition in which the stay  4 B is restrained from moving in an inserting direction, so as to be locked The condition in which the stay  4 B is restrained from moving in the inserting direction can be canceled by disengaging the engagement claw St from the engagement groove Bk by pushing the knob Sb. The engagement grooves Bk are formed in a plurality of positions in the axial direction of the stay  4 B. Therefore, height of the headrest  4  can be freely adjusted by appropriately performing pushing operation of the knob Sb. 
     As described above, upon insertion of the stay  4 B, the lower end portion of the stay  4 B and the upper end portion of the inner member  41  are axially connected to each other, thereby providing a condition in which the head portion  42 H of the outer member  42  of the operation cable  40  is inserted into the cylindrical stay  4 B from below. As a result, the head portion  42 H of the outer member  42  is axially positioned closer to the lower end portion of the push rod  50  that is inserted in the cylindrical stay  4 B. 
     Strictly speaking, a small gap is formed between the lower end portion of the push rod  50  and the head portion  42 H such that the push rod  50  cannot be erroneously pushed by the head portion  42 H of the outer member  42  when the stay  4 B is inserted. 
     Further, in a condition in which the head portion  42 H of the outer member  42  and the lower end portion of the push rod  50  are axially positioned closer to each other, the head portion  42 H and the push rod  50  is in an axial connection condition in which the operational force produced from the lower end of the operation cable  40  being pulled can be reversed by the head portion  42 H of the outer member  42  and be transmitted to the push rod  50  as the pushing operational force. 
     Further, the above-described headrest  4  can be detached from the seat back  2  by pulling the stays  4 B and  4 B from the cylindrical the supports  25  and  2 S. At this time, as the stays  4 B and  4 B are drawn out, an axial connection condition of the inner member  41  and the lower end portion of the stay  4 B as described above is canceled. 
     In particular, as will be recognized from  FIG. 13 , as the stay  4 B is upwardly pulled from the support  2 S, the engagement projections  41 P and  41 P are circumferentially rotated while guided by the curved portion of the insertion grooves Sd and Sd of the support  2 S. As a result, the engagement projections  41 P and  41 P are pushed back from a condition in which the engagement projections  41 P and  41 P are retained in the terminal end positions of the reception grooves Bd and Bd and are moved in such a direction as to be removed from this condition. When the engagement projections  41 P and  41 P move along the curved portions of the insertion grooves Sd and Sd and reach the terminal end positions thereof, the engagement projections  41 P and  41 P are pushed back to axially extended portions of the reception grooves Bd and Bd. As a result, as shown in  FIG. 12 , the engagement projections  41 P and  41 P are transferred again from the condition in which they are suspended from the stay  4 B to the condition in which they are suspended from the support  2 S. Thus, the axial connection condition of the head portion  42 H of the outer member  42  of the operation cable  40  and the push rod  50  in which they are positioned closer to each other can be canceled. 
     Next, the headrest moving mechanism  10  is described. While construction of the headrest moving mechanism  10  is shown in  FIGS. 5 to 11 , the construction is best shown in  FIG. 8 . Therefore, the construction is described with reference to this drawing. 
     The headrest moving mechanism  10  is arranged such that the support portion  4 A is connected to the headrest base portion  4 C. The headrest moving mechanism includes a pair of linking members  12  and  12  that are laterally positioned, support members  13  and  13 , hooks  14  and  14 , an engagement-disengagement member  15 , the tension spring  16  and prevention levers  17  and  17 . 
     The headrest base portion  4 C is made of a synthetic resin. The headrest base portion  4 C has a plate-shaped rear surface portion  11 B, a bottom surface portion  11 D, side surface portions  11 S and  11 S and a top surface portion  11 U that are integrally formed. In particular, the bottom surface portion  11 D extends forwardly from a lower end edge of the rear surface portion  11 B. Further, the side surface portions  11 S and  11 S are vertically positioned on widthwise both sides of the headrest base portion  4 C. Further, the top surface portion  11 U connects upper edges of the side surface portions  11 S and  11 S. 
       FIG. 11  is a view that is viewed from line XI of  FIG. 8 , that is, a front view of the headrest  4  that is viewed from obliquely below. As shown in the drawing, a plurality of upright plate-shaped ribs  11 R—(A symbol “—” means a plural number.) are formed between the side surface portions  11 S and  11 S of the headrest base portion  4 C, so as to reinforce the headrest base portion  4 C. The ribs  11 R—are vertically positioned in parallel. 
     Upper end portions of the stays  4 B and  4 B are respectively inserted into the bottom surface portion  11 D of the headrest base portion  4 C, and are integrally fixed thereto. Further, the stays  4 B and  4 B have tubular shapes and are secured to the bottom surface portion  11 D such that opened upper end portions thereof are exposed to an upper surface side of the bottom surface portion  11 D. 
     Further, wave-shaped elongated holes  11 H are formed in the side surface portions  11 S and  11 S of the headrest base portion  4 C. The elongated holes  11 H and  11 H are formed by cutting out the side surface portions  11 S and  11 S in a thickness direction thereof. The elongated holes  11 H and  11 H have first stopper grooves H 1  and second stopper grooves H 2  that are formed between lower end portions H 0  and H 0  and upper end portions H 3  and H 3 . The first stopper grooves H 1  and the second stopper grooves H 2  are depressed rearwardly (rightwardly in the drawings) in a wave-like and step-like fashion. Further, each of the first stopper grooves H 1  and the second stopper grooves H 2  correspond to a stopper groove of the present invention. 
     Next, referring to  FIG. 8  again, a pair of linking members  12  and  12  are made of a synthetic resin. The linking members  12  and  12  may function as connection members that link the headrest base portion  4 C and the support portion  4 A to each other. The linking members  12  and  12  are positioned on the support portion  4 A so as to be laterally spaced from each other, and their end portions are respectively connected to an upper end portion of the headrest base portion  4 C and a rear surface of the support portion  4 A. 
     In particular, the rear end portions of the linking members  12  and  12  are rotatably supported by a connection shaft  12 A that passes through the side surface portions  11 S and  11 S of the headrest base portion  4 C. Further, as shown in  FIG. 11 , the rear end portions of the linking members  12  and  12  are positioned between the side surface portions  11 S and  11 S and the ribs  11 R and  11 R positioned inside thereof. Thus, the linking members  12  and  12  are rotatably supported by the connection shaft  12 A that extends between the side surface portions  11 S and  11 . 
     Referring to  FIG. 8  again, the front end portions of the linking members  12  and  12  are rotatably supported by a connection shaft  12 B that is laterally positioned on the rear surface of the support portion  4 A. The connection shafts  12 A and  12 B are positioned so as to be parallel to each other. 
     The above-described linking members  12  and  12  are arranged, when rotated clockwise about the connection shaft  12 A that rotatably supports the rear end portions thereof, to contact the top surface portion  11 U of the headrest base portion  4 C, so that their clockwise rotation can be restrained. 
     Next, a pair of support members  13  and  13  are integrally connected to the support portion  4 A so as to extend rearwardly from the rear surface of the support portion  4 A in arm-like fashion. The support members  13  and  13  are positioned on the support portion  4 A so as to be laterally spaced from each other. The support portion  4 A is made of a synthetic resin. Further, the support portion  4 A is integrally formed such that a front surface thereof has a curved plate-shape. Further, the rear surface of the support portion  4 A is integrally formed with support portions for rotatably supporting the connection shaft  12 B. Also, the rear surface of the support portion  4 A is integrally formed with the support members  13  and  13 . 
     Rear end portions of the support members  13  and  13  are connected to each other by a connection shaft  13 A that extends in a width direction. In particular, as shown in  FIG. 11 , the rear end portions of the support members  13  and  13  are positioned between the ribs  11 R and  11 R positioned outside thereof and the ribs  11 R,  11 R positioned inside thereof. Further, the connection shaft  13 A that connects the rear end portions of the support members  13  and  13  is positioned so as to be parallel to the connection shaft  12 A and the connection shaft  12 B described above. Further, the connection shaft  13 A corresponds to an engagement portion of the present invention. 
     End portions of the connection shaft  13 A are passed through the elongated holes  11 H and  11 H that are formed in the side surface portions  11 S and  11 S of the headrest portion  4 C. Therefore, the connection shaft  13 A is capable of moving back and forth and up and down within a range defined by the profiles of elongated holes  11 H and  11 H. Further, the ribs  11 R—( FIG. 11 ) that are formed between the side surface portions  11 S and  11 S are shaped so as to not interfere with the connection shaft  13 A that moves within the elongated holes  11 H and  11 H. 
     Next, as shown in  FIG. 6 , a pair of hooks  14  and  14  are formed to cam shapes as a whole and are disposed in the headrest base portion  4 C. The hooks  14  and  14  are positioned at a lower end portion of the headrest base portion  4 C so as to be laterally spaced from each other. The hooks  14  and  14  are provided as retainer members for retaining the connection shaft  13 A that is capable of moving within the elongated holes  11 H and  11 H at the lower end portions H 0  and H 0  which define an initial position of the connection shaft  13 A. 
     In particular, as shown in  FIG. 11 , the hooks  14  and  14  are positioned between the side surface portions  11 S and  11 S and the ribs  11 R and  11 R positioned inside thereof and are respectively rotatably supported by connection shafts  14 A and  14 A that extend therebetween. 
     Referring to  FIG. 6  again, formed in each of the hooks  14  and  14  is a claw-shaped upper jaw portion  14 B and a claw-shaped lower jaw portion  14 C that extend radially outwardly thereof. Each of the upper jaw portion  14 B and the lower jaw portion  14 C is positioned in two positions on outer circumferential portions of the hook  14 . Formed between the upper jaw portions  14 B and the lower jaw portions  14 C are recesses that are shaped such that the above-mentioned connection shaft  13 A can be receive therein. The connection shafts  14 A and  14 A are positioned so as to be parallel to the connection shafts  12 A and  12 B and the connection shaft  13 A. 
     Further, torsion springs  145  and  14 S are disposed between the hooks  14  and  14  and the headrest base portion  4 C described above. The torsion springs  145  and  14 S are disposed while they are previously twisted. The hooks  14  and  14  are biased by the torsion springs  145  and  14 S so as to be rotated counterclockwise from a position shown in  FIG. 6 . 
     Further, step-shaped engagement grooves  14 D are formed in the outer circumferential portions of the hooks  14  and  14 . The engagement grooves  14 D and  14 D engage a pair of engagement arm portions  15 C and  15 C that are formed in the engagement-disengagement member  15  which will be described hereinafter. Thus, the hooks  14  and  14  can be maintained in a condition in which counterclockwise rotation thereof by biasing forces is restrained. 
     Therefore, in the condition in which the counterclockwise rotation of the hooks  14  and  14  is restrained, the hooks  14  and  14  can maintain the connection shaft  13 A in a condition in which the connection shaft  13 A is retained in the lower end portions H 0  and H 0  of the elongated holes  11 H and  11 H, which portions define the initial position of the connection shaft  13 A, while the connection shaft  13 A is received in the recesses formed between the upper jaw portions  14 B and  14 B and the lower jaw portions  14 C and  14 C. 
     As shown in  FIG. 5 , the connection shaft  13 A is normally biased toward the connection shaft  12 A by the tension spring  16  that is disposed between the connection shaft  13 A and the connection shaft  12 A, so as to be biased toward the upper end portions H 3  and H 3  along the profiles of the elongated holes  11 H and  11 H. Thus, the connection shaft  13 A is normally maintained in an initial condition in which the connection shaft  13 A is retained in the lower end portions H 0  and H 0  of the elongated holes  11 H and  11 H by the hooks  14  and  14  against the biasing force of the tension spring  16 . 
     Further, referring to  FIG. 6  again, when the engagement arm portions  15 C and  15 C are disengaged from the hooks  14  and  14  upon counterclockwise rotation thereof, the hooks  14  and  14  can rotate counterclockwise by biasing forces of the torsion springs  145  and  14 S. As a result, as indicated by broken lines in  FIG. 6 , the upper jaw portions  14 B and  14 B of the hooks  14  and  14  are moved out of the elongate holes  11 H and  11 H, and the lower jaw portions  14 C and  14 C are pushed up from below, so as to be exposed to the elongated holes  11 H and  11 H. Thus, the condition in which the connection shaft  13 A is retained by the hooks  14  and  14  is canceled. As a result, as shown in  FIGS. 7 and 8 , the connection shaft  13 A moves forwardly and upwardly along the profiles of the elongated holes  11 H and  11 H by the biasing force of the tension spring  16 . As a result, the support portion  4 A relatively moves forwardly and upwardly with respect to the headrest base portion  4 C while rotating the linking members  12  and  12 . 
     Referring to  FIG. 6  again, the engagement arm portions  15 C and  15 C that can restrain the counterclockwise rotation of the hooks  14  and  14  are positioned so as to be laterally spaced from each other, and are positioned so as to be capable of engaging the hooks  14  and  14 . In particular, as shown in  FIG. 11 , similar to the hooks  14  and  14 , the engagement arm portions  15 C and  15 C are positioned between the side surface portions  11 S and  11 S and the ribs  11 R and  11 R positioned inside thereof. Further, the engagement arm portions  15 C and  15 C are rotatably supported by a connection shaft  15 B that extends between the side surface portions  11 S and  11 S. The connection shaft  15 B is integrally connected to the engagement arm portions  15 C and  15 C and is rotatably supported by the side surface portions  11 S and  11 S. The connection shaft  15 B is positioned so as to be parallel to the connection shafts  12 A and  12 B and the connection shaft  13 A. 
     Further, a torsion spring  15 S is dispose between one of the engagement arm portions  15 C and the headrest base portion  4 C. The torsion spring  15 S is disposed while it is previously twisted. As shown in  FIG. 6 , the torsion spring  15 S biases the engagement arm portions  15 C and  15 C clockwise. Thus, the engagement arm portions  15 C and  15 C are normally maintained in a condition in which they are pressed to the outer circumferential portions of the hooks  14  and  14  while distal ends of the engagement arm portions  15 C and  15 C respectively engage the step-shaped engagement grooves  14 D and  14 D. When the engagement arm portions  15 C and  15 C engage the engagement grooves  14 D and  14 D, the engagement arm portions  15 C and  15 C and the engagement grooves  14 D and  14 D are oppositely contact each other, so that biased rotation thereof are mutually restrained. 
     Further, referring to  FIG. 5  again, an operation arm portion  15 A is integrally connected to an end portion of the connection shaft  15 B that is connected to the engagement arm portions  15 C and  15 C described above. The operation arm portion  15 A is arranged so as to be rotated by the push rod  50  described above. When the vehicle back-side collision happens and the push rod  50  is pushed upwardly, the operation arm portion  15 A is rotated counterclockwise. As a result, as shown in  FIG. 6 , the operation arm portion  15 A rotates the engagement arm portions  15 C and  15 C in the same direction, thereby disengaging the engagement arm portions  15 C and  15 C from the hooks  14  and  14 . Thus, a condition in which the support portion  4 A is retained in the initial position can be canceled, so that the support portion  4 A is transferred forwardly and upwardly by the biasing force of the tension spring  16 . 
     As shown in  FIG. 8 , forward and upward movement of the support portion  4 A is restrained and stopped when the connection shaft  13 A reaches the upper end portions H 3  and H 3  of the elongated holes  11 H and  11 H. Further, in a condition in which an advancing movement of the support portion  4 A is stopped, the support portion  4 A cannot be pushed back rearwardly even if the support portion  4 A is applied with a loading from the head of the sitting person. 
     That is, when the connection shaft  13 A reaches the upper end portions H 3  and H 3  of the elongated holes  11 H and  11 H, the linking members  12  and  12  connected to the supported portion  4 A contact the top surface portion  11 U of the headrest base portion  4 C, so as to become a condition in which clockwise rotation thereof is restrained. In the condition in which the clockwise rotation of the linking members  12  and  12  is restrained, when the support portion  4 A is applied with a force that urges the same to move rearwardly caused by the rearward inclination of the head, the linking members  12  and  12  are applied with a force that urges the same to press against the top surface portion  11 U of the headrest base portion  4 C. The force thus produced may function as a force that further presses the connection shaft  13 A in an advancing direction (upwardly) from the upper end portions H 3  and H 3  of the elongated holes  11 H and  11 H. Therefore, even if the support portion  4 A is applied with a pressing force as described above, the support portion  4 A is prevented from being rotated counterclockwise. As a result, the support portion  4 A can catch the head of the sitting person in the collision preparatory position. 
     Further, as shown in  FIG. 7 , the support portion  4 A can be prevented from being pushed back rearwardly if it is pressed by the head of the sitting person while it is moving forwardly. That is, when the connection shaft  13 A is applied with a force that urges the same to move rearwardly while the support portion  4 A is moving forwardly, the connection shaft  13 A can enter the first stopper grooves H 1  and H 1  or the second stopper grooves H 2  and H 2  that are formed in the elongated holes  11 A and  11 A so as to be depressed rearwardly (rightwardly in the drawings) in the step-like fashion. As a result, rearward movement of the connection shaft  13 A is restrained, so that the support portion  4 A can be maintained in positions on the way of forward movement thereof. Therefore, even when the support portion  4 A does not yet reach the collision preparatory position, the head of the sitting person can be caught by the support portion  4 A. Further,  FIG. 7  shows a condition in which the connection shaft  13 A is placed in the second stopper grooves H 2  and H 2 . 
     Referring to  FIG. 5  again, attached to the headrest base portion  4 C are a pair of prevention levers  17  and  17 . The prevention levers  17  and  17  are positioned so as to be laterally spaced from each other, and their rear end portions are respectively rotatably connected to the headrest base portion  4 C. 
     In particular, as shown in  FIG. 11 , the rear end portions of the prevention levers  17  and  17  are positioned between the outer ribs  11 R and  11 R and the inner ribs  11 R and  11 R and are respectively rotatably supported by connection shafts  17 A and  17 A that extend therebetween. 
     Further, torsion springs  17 S and  17 S are disposed between the prevention levers  17  and  17  and the headrest base portion  4 C. Further, the torsion springs  17 S and  17 S correspond to spring members of the present invention. As shown in  FIG. 5 , the torsion springs  17 S and  17 S are wound around the connection shafts  17 A and  17 A. One end of each of the torsion springs  17 S and  17 S is connected to each of the prevention levers  17  and  17 . The other end of each of the torsion springs  17 S and  17 S is connected to the headrest base portion  4 C. Thus, in their free conditions, the prevention levers  17  and  17  are maintained in a postural condition in which they are exposed to the elongated holes  11 H and  11 H by spring forces of the torsion springs  17 S and  17 S. 
     Spoon-shaped receiving portions  17 B are respectively formed in left or forward end portions of the prevention levers  17  and  17  that are exposed to the elongated holes  11 H and  11 H. As shown in  FIG. 7 , when the connection shaft  13 A moves upwardly in the advancing direction from the lower ends H 0  and H 0  within the elongated holes  11 H and  11 H, the receiving portions  17 B and  17 B are pushed away by the connection shaft  13 A, so as to be pushed out of the elongated holes  11 H and  11 H. However, as shown in  FIG. 8 , when the connection shaft  13 A reaches the upper end portions H 3  and H 3  of the elongated holes  11 H and  11 H, the receiving portions  17 B and  17 B are returned to the postural condition in which they are exposed to the elongated holes  11 H and  11 H by the spring forces of the torsion springs  17 S and  17 S. 
     Further, as shown in  FIG. 9 , when the connection shaft  13 A is transferred within the elongated holes  11 H and  11 H downwardly in a retracting direction from the upper end portions H 3  and H 3 , the receiving portions  17 B and  17 B catch the connection shaft  13 A by their spoon-shaped distal ends. Further, when, in this condition, the connection shaft  13 A is further transferred downwardly, the prevention levers  17  and  17  are pushed and rotated counterclockwise in the drawing while pressed by the connection shaft  13 A that is caught by the receiving portions  17 B and  17 B. 
     As shown in  FIG. 10 , the connection shaft  13 A, when transferred to a portion closer to the lower end portions H 0  and H 0  while guided by the prevention levers  17  and  17 , is disengaged from the receiving portions  17 B and  17 B. Thus, the connection shaft  13 A, when transferred downwardly from the upper end portions H 3  and H 3  within the elongated holes  11 H and  11 H, is smoothly transferred to the lower end portions H 0  and H 0  beyond the first stopper grooves H 1  and H 1  and the second stopper grooves H 2  and H 2  while guided by the prevention levers  17  and  17 , so as to not enter the first stopper grooves H 1  and H 1  or the second stopper grooves H 2  and H 2 . 
     Further, because the connection shaft  13 A is operated so as to be pressed toward the lower end portions H 0  and H 0  of the elongated holes  11 H and  11 H, the connection shaft  13 A reaches the lower end portions H 0  and H 0  while pressing down the lower jaw portions  14 C and  14 C of the hooks  14  and  14  that are exposed to the lower end portions H 0  and H 0 . As a result, as shown in  FIG. 6 , the hooks  14  and  14  are rotated clockwise in the drawing, so as to be placed in a postural condition in which the upper jaw portions  14 B and  14 B thereof are positioned over the upper side of the connection shaft  13 A. 
     The hooks  14  and  14 , when placed in the condition as described above, engage the engagement arm portions  15 C and  15 C, so as to be is locked again in a condition in which the connection shaft  13 A is maintained in the initial position. As a result, the support portion  4 A is maintained again in a condition in which it is returned to the posture of the initial position thereof, i.e., a position before it is moved forwardly. 
     Next, a method of using the embodiment is described. 
     Referring to  FIG. 1 , in the normal condition in which the vehicle back-side collision does not yet happen, the vehicle seat  1  is in a condition in which the support portion  4 A of the headrest  4  is maintained in the posture of the initial position thereof. When the vehicle back-side collision happens, the pressure receiving portion  21  is pressed rearwardly by the seat back loading applied by the sitting person. As a result, a corresponding operational force is transferred via the operation cable  40  and the push rod  50 , so that the engagement-disengagement member  15  is rotated. 
     Thus, the support portion  4 A is released from a condition in which it is retained in the initial position. As a result, as shown in  FIG. 8 , the support portion  4 A moves to the collision preparatory position by the biasing force of the tension spring  16 . Further, the support portion  4 A that is moved to the collision preparatory position can receive the head of the sitting person that is inclined rearwardly by the impact of the collision from a back-side of the head. 
     Thus, according to the vehicle seat of the embodiment, due to simple construction in which the support portion  4 A is connected to the headrest base portion  4 C and slidably engages the guide pathways (the elongated holes  11 H and  11 H), the support portion  4 A can move from the initial position to the collision preparatory position that can prevent the rearward inclination of the head while maintaining an inclined posture of the support portion  4 A with respect to the headrest base portion  4 C. 
     Further, a stopper mechanism is provided. The stopper mechanism can prevent the support portion  4 A from being pushed back even if the support portion  4 A is applied with a loading from the head when the support portion  4 A reaches the collision preparatory position. The stopper mechanism is constructed of an engagement structure in which the support portion  4 A and the headrest base portion  4 C are slidably engaged with each other. Therefore, construction for preventing the rearward inclination of the head can be simplified and rationalized. 
     Further, formed in the guide pathways (the elongated holes  11 H and  11 H) along which the support portion  4 A slides are the stopper grooves (the first stopper grooves H 1  and H 1  and the second stopper grooves H 2  and H 2 ) that are capable of preventing the support portion  4 A from being pushed back by the head. Therefore, even when the support portion  4 A is moving in an advancing direction, the head can be prevented from being inclined rearwardly. As a result, such construction can be simplified. 
     Further, an engagement prevention mechanism (the prevention levers  17  and  17  and the torsion springs  17 S) is provided. When the support portion  4 A is moving in the advancing direction, the engagement prevention mechanism allows the stopper portion  4 A to engage the stopper grooves (the first stopper grooves H 1  and H 1  and the second stopper grooves H 2  and H 2 ) on the way in an advancing movement thereof. To the contrary, when the support portion  4 A is moving in a retracting direction from the collision preparatory position, the engagement prevention mechanism is capable of guiding the support portion  4 A such that the support portion  4 A can pass over the stopper grooves (the first stopper grooves H 1  and H 1  and the second stopper grooves H 2  and H 2 ). Therefore, the support portion  4 A can be smoothly returned to the initial position. 
     Further, the guide pathways (the elongated holes  11 H and  11 H) extend forwardly and upwardly so as to correspond to a moving direction of the support portion  4 A. Therefore, the support portion  4 A can move while substantially maintaining the inclined posture thereof. As a result, a supporting posture of the support portion for supporting the head can be maintained. 
     An embodiment of the present invention is described hereinbefore. However, the present invention can be carried out in various forms. 
     For example; the linking members that link the headrest base portion and the support portion can be arranged and shaped as shown in  FIGS. 15 and 16 . Also, the elongated holes as the guide pathways can be arranged and shaped as shown in  FIGS. 15 and 16 . In particular, as shown in  FIG. 15 , the linking members and the elongated holes can be arranged so as to cross each other. Conversely, as shown in  FIG. 16 , the linking members can be pivotally connected at positions closer to a lower end of the headrest base portion, so that the elongated holes can be positioned above the linking members. Further, in these modified forms, the linking members are arranged outside of the headrest base portion. However, it is not limited to such arrangement. 
     Further, as disclosed in Japanese Laid-Open Patent Publication No. 2005-104259, the headrest moving mechanism can be constructed such that when the cables are moved by a desired distance, the support portion of the headrest can directly move in the advancing direction by a distance corresponding to the moving distance of the cables. 
     Further, the guide pathways formed in the headrest base portion are not limited to the elongated holes formed as through grooves as shown in the above-described embodiment. That is, the guide pathways may have various forms that can slidably receive the support portion. In particular, the guide pathways can be formed as bottomed grooves that are formed in the headrest base portion so as to be depressed in a width direction thereof. Further, the guide pathways can be formed as rail-shaped ribs that are formed in the headrest base portion so as to slidably guide the support portion. 
     Further, in the embodiment, the guide pathways extend forwardly and upwardly so as to correspond to the moving direction of the support portion. However, the guide pathways can be arranged so as to extend forwardly or upwardly. However, it must be noted that the inclined posture of the support portion can be easily changed when the support portion moves in the advancing direction or the retracting direction. 
     Further, the stopper grooves formed in the guide pathways can be a single stopper grooves. Conversely, the stopper grooves can be three or more stopper grooves. Further, the stopper grooves must be arranged such that when the support portion is retracted, the engagement portion (the connection shaft) of the support portion can be moved beyond all of the stopper grooves as the prevention levers are rotated.