Abstract:
A vehicular child safety seat includes a seat main body ( 1 A) attached to a base body ( 1 C) in a rotatable manner. The seat main body includes a seat portion ( 16 ), a backrest ( 11 ) reclinably hinged to the seat portion, a head guard ( 10 ) tiltably hinged to the upper edge of the backrest, side guards ( 12, 13 ) protruding perpendicularly from lateral edges of the backrest, and armrests ( 14, 15 ) protruding upwardly along side edges of the seat portion. When the backrest is in its most reclined position, and the seat main body is rotated on the base body to be transverse to the longitudinal direction of the vehicle, the seat portion and backrest are substantially coplanar and horizontal to form a baby bed. When the backrest is in a more upright position, and the seat main body is rotated to face forward or rearward in the vehicle, the safety seat is suitable for an infant, toddler or older child sitting upright. Thus, a vehicular child safety seat can be provided that can be used constantly throughout the entire childhood for a baby, an infant, and a child of school age.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is related to copending U.S. application Ser. No. 09/238,265, having the same Assignee as the present application. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a vehicular child safety seat placed on a seat of a vehicle and fastened to the seat of the vehicle using a seat belt with which the vehicle is equipped. 
     2. Description of the Background Art 
     In recent years safety measures have emerged as a critical factor for vehicles, and various vehicular child safety seats have been developed as a device for insuring the safety of a new-born baby, a baby, an infant, and a child of school age (referred to generically as “child” hereinafter) riding in a vehicle. 
     The Japanese Industrial Standard has established detailed standards in “Vehicle Equipment-Child Protection Device: JIS-D 0401-1996” with respect to a vehicular child safety seat. 
     According to the Japanese Industrial Standard, the type of a child protection device or child restraint is classified into “baby (including new-born baby) bed”, “infant seat (for both forward and rear facing directions)” and “child seat”. The classification is divided according to the weight range of the child, i.e., W1 (less than 10 kg), W2 (at least 9 kg and less than 18 kg), W3 (at least 15 kg and not more than 25 kg), and W4 (at least 22 kg and not more than 36 kg). In order to satisfy the safety requirements for the classes of W1 and W2, an infant seat, when used in a rear facing direction with respect to the direction of travel, must have a mechanism to prevent the tilted angle of the backrest, i.e. the angle of the surface of the backrest relative to the vertical plane, from exceeding the maximum of 60 degrees at the time of a crash accident on account of the rebound of the infant seat. 
     In general, the child protection apparatus for weight class W1 refers to an apparatus fixed to the seat of a vehicle in a direction lateral or facing rearward with respect to the direction of travel of a vehicle for babies aged 0-10 months. The child protection apparatus for weight class W2 refers to an apparatus fixed to a seat of a vehicle in a frontward or backward facing direction with respect to the direction of travel of the vehicle for infants from 6 months to 3 or 4 years (commercially available as a “child seat” in general). The child protection apparatus for weight class W3 refers to an apparatus for elevating the seat bottom plane for a child aged 3 or 4 years to 6 years (commercially available as a “booster” in general). 
     Various products have been developed as devices to be applied to vehicular child protection apparatuses for weight W1, W2 and W3. 
     The rate of usage of child protection apparatuses in vehicles in the Japanese society is extremely lower than that compared to US and European countries. The first factor can be ascribed to the lack of the parents in recognizing the danger associated with vehicles. Furthermore, if a child shows his/her reluctance in being seated in the child protection apparatus, there is a possibility that the child protection apparatus will not be used. These all come from the lack of danger appreciated by adults associated with vehicles. 
     In view of the foregoing, it is first necessary to reform the recognition of adults as to the danger associated with automobiles. Aggressive official activities by the government are expected. The problem of a child showing reluctance to use a child protection apparatus can be obviated more or less by always having a child seated in the child protection apparatus ever since his/her babyhood, i.e. before a baby begins to take notice, so that the child will take it for granted that a child protection apparatus must be used when riding in a car. 
     This is extremely important from the social point of view. Thus, a child protection apparatus that can be used consistently from babyhood to school age is desired in order to inculcate the habit and recognition of always using a child protection apparatus for children riding in a car. In other words, a child protection apparatus is desired that commonly corresponds to the specification of all the above-described weight classes W1 to W3. 
     However, child protection apparatuses currently available on the market are only solitary products for a respective single weight class W1, W2, or W3 independently, or a product that has a rotatable seat and a reclinable backrest as disclosed in Japanese Patent Laying-Open No. 1-113007. There is no product that can be commonly used for weight classes W1 and W2, or for weight classes W1 to W3. Although the vehicle infant restraint seat disclosed in the aforementioned Japanese Patent Laying-Open No. 1-113007 seems to be available for consistent usage for W1 and W2 in common, this seat apparatus is intended for only infants and children. It is not suitable for small babies and infants, particularly those in supine and prone positions. This is because the apparatus, when taking a bed-like flat position by reclining the backrest portion, has its head portion open with inadequate support for babies to be laid on the apparatus. This restraint seat does not have the requirements for weight class W1. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a vehicular child safety seat that can be consistently used for children of various ages from babyhood to childhood. 
     According to an aspect of the present invention, a vehicular child safety seat is fixed to a seat of a vehicle using a seat belt with which the vehicle is equipped, and has a support plane for supporting a child. The support plane is provided in a selectable manner to be configurable into a first state forming a bed plane enclosing the head and side portions of a child for laying the child transverse to the direction of travel of the vehicle, and a second state forming a seat plane for seating the child facing forward or rearward with respect to the direction of travel of the vehicle. 
     According to the vehicular child safety seat of the above-described structure, the child can be laid transverse to the direction of travel of the vehicle. Usage corresponding to weight class W1 is allowed by forming a bed plane enclosing the head and side portions of the child. Also, usage for weight class W2 is allowed by forming a seat plane for seating the child facing frontward or rearward to the direction of travel of the vehicle. 
     Thus, the vehicular child safety seat of the present invention can be used in common from babyhood to childhood. 
     According to another aspect of the present invention, a vehicular child safety seat is implemented by a seat device including a seat bottom, a backrest arranged to be reclinable with respect to the seat bottom, a pair of side guards extending from both sides of the backrest and substantially perpendicular to the front side surface of the backrest, a head guard attached at the upper end of the backrest in a manner so as to be angularly orientable relative to the front side surface of the backrest. 
     According to the above structure, the top and sides of the head and the sides of the body of a child can be protected in a safe and comfortable state to improve the safety of the child in a vehicle. 
     According to a further aspect of the present invention, the above vehicular child safety seat further includes a support device and is a base. The support device includes a pair of arm rests at the inner side and in close proximity to the pair of side guards for supporting the pair of side guards in a backward and frontward rotatable manner about the support point, and a connection plate connecting the lower ends of the pair of arm rests. The base includes a holding device for holding the support device in a rotatable manner to allow rotation in a substantially horizontal direction, i.e. on a substantially horizontal rotation plane. 
     According to the above structure, the sides of the body of the child can be protected sufficiently. Also, by the reclinable backrest and the support device rotatable with respect to the base, the support device can be oriented in a direction transverse, forward, or rearward with respect to the direction of travel of the vehicle. 
     In the above vehicular child safety seat, the seat device is detachable from the base. By this structure, the vehicular child safety seat can be used for weight class W3. 
     In the above vehicular child safety seat, the holding device preferably includes a circular convex portion provided at the base, a through opening provided in the connection plate for receiving the convex portion, and a support plate sandwiching the connection plate with the base and secured to the convex portion so that the connection plate is rotatable. 
     By making the support device rotatable with respect to the base, the support device can be implemented in a more preferable form in the transverse, forward or backward direction with respect to the direction of travel of the vehicle. 
     In the above vehicular child safety seat, the holding device and the backrest preferably have a backrest inclination limit device provided to allow the backrest to be reclined when the seat device is in a transverse position, and to prevent the backrest from reclining when at positions other than the transverse position of the seat device. 
     The backrest inclination limit device also embodies a mechanism for preventing the tilted angle of the backrest position, i.e. the angle between the surface of the backrest and the vertical direction, from exceeding the maximum of 60 degrees during a crash accident taking into consideration the rebound of the vehicular child safety seat at the time of collision when the vehicular child safety seat is used as a backward facing infant seat. 
     In the above vehicular child safety seat, the backrest inclination limit device preferably includes an engagement projection provided in the proximity of the lower end portion of the backrest at the rear side, a concave groove provided at the support plate extending in the lateral direction that can receive the engagement projection so that the backrest can be inclined when the seat device is in the transverse state, and an abutment plane for preventing the reclining of the backrest by abutting against the engagement projection when an attempt is made to recline the backrest in a state other than in the transverse state. 
     According to the above structure, the space of the vehicular child safety seat can be used effectively. Also, when the vehicular child safety seat is used as a rearward facing infant seat, the mechanism can be realized to prevent the tilted angle of the backrest portion from exceeding the maximum of 60 degrees at the time of a collision taking into consideration the rebound of the vehicular child safety seat. 
     In the above vehicular child safety seat, the engagement projection is preferably provided so as to be received in the concave groove to prevent the rotation of the seat device relative to the base when the seat device is in a transverse state. According to this structure, rotation of the seat device relative to the base can be prevented when the seat device is in a transverse state used as a bed. Thus, the safety can be improved when the device is used as a bed. 
     According to still another aspect of the vehicular child safety seat of the present invention, the pair of side guards and the pair of armrests are arranged to have a portion thereof overlapping each other. By this structure, the side of the child can be protected by a double structure of at least the side guard and the armrest. Particularly in the case where the vehicular child safety seat is used as a bed in the transverse state, the safety at the time of a crash accident can be improved. 
     According to yet a further aspect of the vehicular child safety seat of the present invention, the seat bottom is supported by the pair of armrests so that the seat bottom slides toward the backrest as the backrest is moved upwards. According to this structure, the seat bottom slides backward as the backrest is moved to its upright position correspondingly as the child is transposed to a sitting position from a lying position. Therefore, any drag or pulling of the clothing and the like at the back of the child is eliminated. The child can be moved from a lying position to a sitting position in a natural manner. Preferably, the above vehicular child safety seat further includes a crotch belt passing through an elongated hole provided in the seat portion along the sliding direction of the seat portion and fastened to the connected plate. By this structure, the distance between the backrest and the crotch belt is increased when the seat portion slides backward as the backrest is moved upright since the crotch belt is fastened to the connection plate. When the crotch belt is used as a 5 point seat belt together with a pair of shoulder belts and a pair of lumbar belts in a preferable embodiment, the child user will feel no oppressive or uncomfortable sensation even when transposed from a lying position to a sitting position. A comfortable posture can be provided. 
     In yet another aspect of the vehicular child safety seat of the present invention, the base further includes a seat fixation device for selectively fixing the rotary orientation of the seat device relative to the base. By this structure, the vehicular child safety seat can be secured in a transverse, forward-facing or backward-facing state with respect to the direction of travel of the vehicle. 
     According to yet another aspect of the vehicular child safety seat of the present invention, a connection device is provided between the pair of armrests and the head guard so that the head guard is oriented substantially perpendicular to the front side surface of the backrest when the backrest is at its most reclined position. By this structure, the head guard will be in an orientation substantially perpendicular to the front side surface of the backrest when the vehicular child safety seat is set to a bed position with the backrest in the most reclined state. The child using the vehicular child safety seat will be enclosed by the head guard, the pair of side guards, and the pair of armrests to be able to be protected at a high safety level. 
     In the above vehicular child safety seat, the connection device preferably includes a connection bar having one end rotatably connected in the proximity of the lower end of the head guard and another end accepted in a rotatable and slidable manner in an elongated guide aperture provided in the armrest. The elongated guide aperture includes a first elongated aperture portion provided to gradually approach the support point so that the head guard is oriented substantially perpendicular to the front side surface of the backrest portion when the backrest is at its most reclined position, and a second elongated aperture portion having an arc shape provided continuous to the first elongated aperture portion and on the circumference centered about the support point so that the head guard gradually becomes substantially flush with the backrest as the backrest is moved to its upright position. By this structure, the head guard that is oriented substantially perpendicular to the front side surface of the backrest when in a bed-like state can be gradually rendered coplanar with the backrest, due to the mechanical structure, while the backrest is transitioned from the most reclined state to an upright state. 
     In another aspect of the vehicular child safety seat of the present invention, the seat device includes a reclining mechanism for selectively altering the tilted angle of the backrest relative to the seat bottom between the position in which the backrest is most reclined with the seat bottom and the backrest substantially in a flat state to form a bed plane on which a child is to be laid, and the position in which the backrest is in the most upright position for the child to take a seating posture. By this structure, the tilted angle of the backrest relative to the seat bottom can be selectively altered to provide a comfortable posture for the child. 
     Preferably, the reclining mechanism includes at least two engagement holes in the armrest, an engagement pin provided at the inner side of the side guard and biased by a resilient member so as to be inserted into the engagement hole in a normal state, and a release handle provided at the backside of the backrest for releasing the bias applied to the engagement pin by the resilient member. By this structure, the tilted angle of the backrest relative to the seat bottom can be selectively altered in a more preferable manner. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view showing a schematic structure of a child seat according to a first embodiment of the present invention. 
     FIG. 2 is an exploded perspective view showing a structure of the child seat according to the first embodiment of the present invention. 
     FIGS. 3-5 show respective use positions of the child seat according to the first embodiment of the present invention. 
     FIGS. 6-8 respectively show different positions of the reclining mechanism of the child seat according to the first embodiment of the present invention. 
     FIGS. 9-11 respectively show a rotation lock device of the child seat according to the first embodiment of the present invention. 
     FIGS. 12-15 respectively show a mechanism to prevent reclining of the backrest of the child seat according to the first embodiment of the present invention. 
     FIGS. 16 and 17 respectively show an entire structure of a child seat according to a second embodiment of the present invention in two different positions. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     A vehicular child safety seat according to a first embodiment of the present invention will be described hereinafter with reference to the drawings. First, the structure of a vehicular child safety seat (simply referred to as “child seat” hereinafter)  1  of the first embodiment will be described with reference to FIGS. 1 and 2. 
     In child seat  1 , the surface of the members constituting child seat  1  are generally covered with a cushioning cover, for example urethane foam, to absorb any impact so as to ensure comfort and safety features. However, child seat  1  is illustrated in the drawings without the urethane foam in the present embodiment for better understanding of the structure of child seat  1 . A child seat with urethane foam will be described in the subsequent second embodiment (FIGS.  16  and  17 ). 
     Referring to FIG. 1, child seat  1  includes a seat device  1 A, a support device  1 B, and a base  1 C. 
     Seat device  1 A includes a seat bottom  16 , and a backrest  11  hingedly connected at the rear end side of seat bottom  16  via a hinge member  18  in a reclinable manner. A pair of side guards  12  and  13  are provided at both sides of backrest  11 , extending substantially perpendicular to backrest  11 . A head guard  10  is connected to the upper end of backrest  11  via a hinge member  19  in a manner so that the head guard is rotatable forwardly and backwardly to an angular orientation. 
     Support device  1 B includes a pair of armrests  14  and  15  arranged at the inner side and in close proximity to the pair of side guards  12  and  13 , and a connection plate  21  (refer to FIG. 2) connecting the lower ends of the pair of armrests  14  and  15 . Base  1 C maintains support device  1 B to be rotatable in the horizontal direction, i.e. on a horizontal rotation plane about a vertical rotation axis by means of a holding device (described subsequently). 
     Child seat  1  includes a crotch belt  17  that extends from beneath the crotch of the child supported in child seat  1  upward to firmly retain the child. A pair of shoulder belts extending from above both shoulders of the child and a pair of lumbar belts supporting the lumbar portion of the child will be described together with crotch belt  17  in the subsequent second embodiment (FIGS.  16  and  17 ). 
     The structure of child seat  1  will be described in further detail with reference to FIG.  2 . 
     Seat device  1 A and support device  1 B are connected by means of a respective pin bolt  34 , a respective washer  35  and a respective nut  36  so as to couple bolt holes  33 ,  33  provided in armrests  14  and  15  as the support points and bolt holes  37 ,  37  in side guards  12  and  13  functioning as supported points. 
     The pair of side guards  12  and  13  and the pair of arm rests  14  and  15  are arranged to have respective portions overlap each other at the general region or vicinity of bolt holes  33 ,  33  and bolt holes  37 ,  37 . 
     By this structure, the side of a child user will be protected by a double structure of at least side guards  12  and  13  and armrests  14  and  15 . The safety of the child seat  1  at the time of a car accident such as a collision or crash when child seat  1  is used as a bed in a transverse position can be improved. 
     Guide protuberances  16   a  and  16   b  are provided at both side faces of seat bottom  16 . Protuberances  16   a  and  16   b  are attached so as to fit respectively into a guide groove  32  of a block  31  provided at the inward side of armrests  14  and  15 . This causes seat bottom  16  to be gradually shifted backward along guide groove  32  when backrest  11  of seat device  1 A pivots forward about pin bolt  34 . 
     Crotch belt  17  is attached to a recess  23  provided in connection plate  21  of support device  1 B by means of a bolt (not shown) and extends through a through opening  16   c  in seat bottom  16 . By this structure, seat bottom  16  slides backward as backrest  11  is moved to its upright position. However, the space between backrest  11  and crotch belt  17  becomes wider since crotch belt  17  is fixed to connection plate  21 . The child will not feel any oppressive or uncomfortably confining sensation even when moved from a lying position to a sitting position where the crotch belt is used in a more preferable manner as a component of a five point seat belt together with the pair of shoulder belts and the pair of lumbar belts (second embodiment: FIGS.  16  and  17 ). Therefore, the child can maintain a comfortable position. 
     An engagement pin  13   b  is attached to a fix plate  13   a  respectively at the inner side of side guards  12  and  13  to adjust the tilted angle of backrest  11 . Engagement pin  13   b  is biased by a spring  13   c  so as to always protrude forward. One end of a wire  13   d  is connected to the trailing end of engagement pin  13   b . The other end of wire  13   d  is threaded through an opening lla in backrest  11  to be connected to a reclining lever  43  (refer to FIG. 6) provided at the backside of backrest  11 . 
     Engagement holes  25   a ,  25   b ,  25   c  and  25   d  for receiving engagement pin  13   b  are provided at the trailing end sides of armrests  14  and  15 . Engagement holes  25   a - 25   d  are provided in a reinforcement plate  29 . Reinforcement plate  29  is provided at the trailing end side of armrests  14  and  15 . 
     In the state where engagement pin  13   b  is fitted in engagement hole  25   d , seat bottom  16  and backrest  11  are substantially flush with each other, taking a horizontal flat state to form a bed plane. In the state where engagement pin  13   b  is fitted into engagement hole  25   a , backrest  11  is at its most upright position. The number of four engagement holes provided in the present embodiment is arbitrary. The tilted angle of backrest  11  can be selected arbitrarily according to the number of the engagement holes. 
     Head guards  10  and the pair of armrests  14  and  15  are connected by a link bar  40  on each side. Link bar  40  has one end connected to a lower end portion  10   a  of head guard  10  in a rotatable manner, and the other end  40   b  fitted into an elongated link guide aperture  24  provided at the trailing end side of armrests  14  and in a rotatable and swingable manner. 
     Head guard  10  is attached at the upper end of backrest  11  and is supported rotatably by a hinge  19   a  secured to backrest  11  by at least one pin  19 . 
     Elongated link guide aperture  24  includes a first aperture portion  24   b  that gradually approaches the support point implemented by bolt holes  33 ,  37  and pin bolt  34  so that head guard  10  is oriented substantially perpendicular to the front side surface of back rest  11  when back rest  11  is at the most reclined position, and an elongated circular second aperture  24   a  provided on the circumference about the support point and continuous to the first elongated aperture  24   b  so that head guard  10  becomes substantially coplanar with backrest  11  as backrest  11  is gradually moved to its upright position. 
     The reclining operation of child seat  1  and the rotatable motion of head guard  10  that cooperate with the reclining operation will be described below. 
     The mounting of support device  1 B that supports seat device  1 A onto base  1 C will be described hereinafter. An opening  22  is provided substantially at the center portion of connection plate  21  of support device  1 B. Support plate  30  is fastened to base  1 C through opening  22  by threading in a screw (not shown) into screw holes  20   b  of base  1 C. Here, support plate  30  is positioned by a convex portion  20   c  provided on fix plate  20   a . Support device  1 B sandwiched by support plate  30  and base  1 C is rotatable in a horizontal plane along a groove  20   g  provided in base  1 C. A seat belt fixation groove  20   f  for engaging the seat belt with which the vehicle is equipped is provided in base  1 C. 
     A lock device insert hole  20   d  for receiving a rotation lock device  50  for immobilizing the rotation of support device  1 B is provided at the front side of base  1 C. Support device  1 B takes up a locked position by threading lock pin  51  of lock device  50  through hole  20   e  to engage with a lock hole  20   i  provided in support device  1 B (refer to FIGS.  9  and  10 ). The details of the mechanism of rotation lock device  50  will be described below. Various use positions of child seat  1  of the above structure are shown in FIGS. 3-5. Child seat  1  is fastened to a seat  120  of a vehicle by means of a seat belt  121  with which the vehicle is equipped. 
     A typical mounted state is shown in FIG.  3 . While base  1 C is fixed to seat  120  by means of seat belt  121 , support device  1 B is rotated 90 degrees relative to base  1 C to take up a transverse state. Backrest  11  is reclined to its full extent, whereby seat device  1 A is configured in a bed state (according to weight class W1 specification). The safety of a child can be maintained with the child taking a lying posture. 
     Referring to FIG. 4, backrest  11  is moved in the upright direction with base  1 C still immobilized. Support device  1 B is rotated relative to base  1 C so as to face the seat device  1 A in the rearward direction. This will allow an infant of about 3 months to about 10 months that can hold his/her head firmly on his/her own to face the person such as the mother seated adjacent thereto. Also, this provides a safety seat position at the time of a crash for babies of this age. FIGS. 4 and 5 correspond to the weight class W2 specification. 
     FIG. 5 shows the state where support device  1 B is fixed to base  1 C so that the occupant faces forward. A comfortable seat is provided for infants of above 10 months. 
     FIGS. 6-8 are diagrams for describing the reclining mechanism of seat device  1 A and the movement of head guard  10  to an upright position in cooperation with the reclining mechanism. 
     FIG. 6 shows the state where backrest  11  is at its fullest reclined state. This forms the optimum bed plane on which a child can be laid. Engagement pin  13   b  fits into engagement hole  25   d  respectively provided in armrests  14  and  15 . The lower end portion  40   b  of link rod  40  is located at the terminal end of first elongated aperture  24   b  of elongated link guide aperture  24 . Therefore, lower end  40   b  is immobilized even when backrest  11  carries out a pivoting motion. 
     In comparison with the state shown in FIG. 8 where backrest  11  takes the most upright position, link rod  40  is urged upwards at the terminal end of first elongated aperture  24   b . Head guard  10  pivots about hinge  19   a  to be oriented substantially perpendicular to backrest  11 . As a result, backrest  11  is enclosed by side guards  12  and  13  and head guard  10 . The safety of the child supported in the child seat can be ensured. 
     In the state shown in FIG. 6, if it is desired to change the reclining angle of the backrest  11 , reclining lever  43  attached to-backrest  11  via a rotary axis  43   a  in a manner so that reclining lever  43  is rotatable in the upward and backward direction is lifted in the direction indicated by arrow A. In response, wire  13   d  is pulled in the direction indicated by arrow B in the drawing, whereby engagement pin  13   b  is pulled out of engagement with engagement hole  25   a . In this state, backrest  11  is pivotable about pin bolt  34 , so that backrest  11  can be moved to a more upright position as shown in FIG.  7 . By releasing reclining lever  43  at this time, engagement pin  13   b  will be fitted into engagement hole  25   c  by the bias of spring  13 . 
     At the same time, seat bottom  16  slides backward along seat guide groove  32 . Lower end portion  40   b  of link rod  40  is located at second elongated aperture portion  24   b  of elongated link guide aperture  24 . Head guard  10  is oriented at a non-perpendicular angle relative to back rest  11  from the state shown in FIG.  6 . 
     To change the state shown in FIG. 7, reclining lever  43  is pulled upward to disengage pin  13   b  from engagement hole  25   c . Backrest  11  is moved to a further upright position. Backrest  11  takes the most upright position when engagement pin  13   b  is fitted into engagement hole  25   a . Seat bottom  16  is further moved backward along elongated seat guide aperture  32 . 
     By providing elongated seat guide aperture  32  so as to gradually extend downward and frontward, seat bottom  16  is tilted frontward to provide a comfortable bed plane for the child. 
     When backrest  11  is to be reclined, the tilted angle of backrest  11  can easily be adjusted by a procedure opposite to that described with reference to FIGS. 6-8. 
     Stoppers  40 S and  10 S that are engaged at the lower ends of a bent portion  40   a  located midway of link rod  40  and head guard  10  are preferably provided so that head guard  10  can be pivoted smoothly and positively when backrest  11  is moved to an upright position. 
     The mechanism of rotation lock device  50  for immobilizing support device  1 B relative to base  1 C will be described hereinafter with reference to FIGS. 9 and 10. 
     FIG. 9 shows the state in which support device  1 B is fastened to base  1 C. A lock pin  51  has its lower end engaged into a guide groove  54  in lock plate  52  and the upper end fitted in a lock hole  20   i  provided in support device  1 B. 
     Lock plate  52  is constantly pulled in the backward direction by the bias of a spring  53  having one end engaged with the rear end of lock plate  52  and the other end anchored to an engagement portion  20   h  provided in base  1 C. As a result, lock pin  51  is always fitted into lock hole  20   i  provided in support device  1 . 
     By pulling out a pull plate  55  provided at the leading end of lock plate  52  in the direction of arrow A, lock pin  51  is shifted gradually in the direction of arrow B along guide groove  54 . In response, the engagement of lock pin  51  with lock hole  20   a  is released as shown in FIG. 10, so that base  1 C can rotate freely. In a normal state, lock pin  51  is fitted into lock hole  20   i , so that rotation lock device  50  always operates to take a safety state. 
     The mechanism for allowing backrest  11  to be reclined when seat device  1 A takes a transverse position, and for preventing backrest  11  from being reclined when seat device  1 A takes a position other than the transverse position will be described hereinafter with reference to FIGS. 11-15. 
     As shown in FIG. 11, an engagement projection  11   e  is provided in the proximity of the lower end at the backside of backrest  11 . Support plate  30  (refer to FIG. 2) includes a concave groove  30   c  that extends transversely for receiving engagement projection  11   e  to allow backrest  11  to be reclined when seat device  1 A takes a transverse position, and abutment faces  30   a  and  30   b  to prevent reclining of backrest  11  by abutting against engagement projection  11   e  when seat device  1 A takes a position other than the transverse state. 
     This mechanism is shown more specifically by FIGS. 12 and 13 which are enlarged partial vertical sectional views of the vicinity of the lower portion of backrest  11  when seat device  1 A takes a forward facing or backward facing state. When an attempt is made to recline backrest  11  in the state shown in FIG. 12, engagement projection  11   e  provided at the lower end of backrest  11  abuts against abutment faces  30   a  or  30   b  at an area A shown in FIG. 13 to prevent backrest  11  from reclining. 
     FIGS. 14 and 15 are partial enlarged vertical sectional views of the vicinity of the lower end of backrest  11  when seat device  1 A is in the transverse state. As backrest  11  is reclined from the state shown in FIG. 14, engagement projection  11   e  provided at the lower end of backrest  11  is received in concave groove  30   c  to allow backrest  11  to be reclined as shown in FIG.  15 . Here, engagement projection  11   e  also has the sidewall portion thereof substantially accommodated in recess groove  30   c , whereby the rotation of seat device  1 A relative to base  1 C is suppressed. 
     This mechanism of preventing reclining of backrest  11  when in the forward facing or backward facing position provides the following advantages. When child seat  1  is used as a backward facing infant seat, the mechanism can be realized of preventing the maximum tilted angle of backrest  11 , i.e. the angle between the surface of the backrest and the vertical plane, from exceeding 60 degrees taking into consideration the rebound of the vehicular child safety seat at the time of a crash. When seat device  1   a  is used as a bed in a transverse state, rotation of seat device  1 A relative to base  1 C can be suppressed to further increase the safety when used in a bed position. 
     Thus, child seat  1  of the present embodiment can meet the safety standards for weight classes W1 and W2 in accordance with the Japanese Industrial Standard (vehicular equipment-child protection apparatus: JIS-D 0401-1996) regulations. 
     Although engagement projection  11   e  is provided at backrest  11  and concave groove  30   c  and abutment faces  30   a  and  30   b  are provided at support plate  30  in the above described structure, a similar function can be implemented by providing a concave groove on backrest  11  and an engagement projection on support plate  30 . 
     In the child seat  1  of the present embodiment, backrest  11  is connected to seat bottom  16  in a reclinable manner, and seat bottom  16  and backrest  11  can form a substantially horizontal plane for a bed when backrest  11  is at its most reclined position. When a child is laid in child seat  1  in this position, the child can be supported in the vehicle in a safe and comfortable position. 
     Second Embodiment 
     A vehicular child safety seat according to a second embodiment of the present invention will be described hereinafter with reference to the drawings. 
     A child seat  100  which is the vehicular child safety seat of the present embodiment has a cushion material attached to child seat  1  of the first embodiment shown in FIG.  1 . Referring to FIGS. 16 and 17, child seat  100  further includes a pair of shoulder belts  114  and  115  and a pair of lumbar belts  117  and  118  to form a 5 point seat belt with crotch belt  17 . The height of the pair of shoulder belts  114  and  115  can be adjusted by means of selecting an appropriate pair of adjustment apertures  125  provided at backrest  11  to allow the body of the child to be supported appropriately according to the body size of the child. In the drawings, components corresponding to those of child seat  1  of the first embodiment are denoted by the same reference characters. 
     FIG. 16 is a perspective view of child seat  100  in the position for use as a bed. FIG. 17 is a perspective view of child seat  100  in the position for use as a seat. 
     Child seat  100  further includes a pair of head guards  107  and  108  that are inclinable toward the inner side to further improve the safety protection of the side of the head of a child. 
     It is to be understood that the embodiments disclosed herein are only examples in all issues, and are not to be limited to the structure and operation shown. For example, the present invention is not limited to the above-described structure of the child seat  1  where support device  1 B is attached to base  1 C so as to be rotatable relative thereto. A structure can be adopted where support device  1 B and base  1 C are detachable, and the orientation of support device  1 B and base  1 C can be altered as necessary. Also, a structure can be adapted where support device  1 B is fastened to base  1 C by providing a seat belt fixation groove  20   f  at two locations crossing each other in base  1 C. 
     The reclining mechanism of backrest  11  is not limited to the disclosed mechanism. For example, the reclining mechanism disclosed in Japanese Patent Laying-Open No. 7-17312, for example, or a reclining mechanism employing various other embodiments well known in the art can be adapted. 
     Since child seat  1  of the present invention is adapted to be used by children of all ages such as a new-born baby to children of school age, appropriate mechanisms for altering the distance between side guards  12  and  13 , the distance between armrests  14  and  15 , and the position of head guard  10  with respect to backrest  11  according to the body size of babies, infants, and children can be adapted to suitably fit the child. 
     Also, a mechanism can be adapted to increase the thickness of the pair of side guards  12 ,  13  and armrests  14  and  15  and also increase the overlapping area between a side guard and an armrest to further improve the safety of a small baby when used in the bed position shown in FIG.  3 . 
     Furthermore, the child seat of the present invention can be used in a manner having seat device  1 A removed and using support device  1 B and base  1 C for weight class W3 and covering support device  1 B with a cushion member and the like. Thus, the child seat of the present invention can be used consistently in common for children corresponding to the weight class W1 specification to the weight class W3 specification. 
     Although not illustrated, the cushion member of FIGS. 16 and 17 can have appropriate grooves provided at the surface thereof to improve the passage of air along the area of the cushion member in contact with the child so as to improve the comfort of the child held in the child seat. 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.