In the conventional support mechanism capable of supporting the distal end of the leg bottom section at either a high position or a low position in the state where the knee bottom section is pivotally raised, the distal end of the leg bottom section that is set at the high position can be shifted to the low position only in the state where the knee bottom section is kept pivotally raised. If the knee bottom section is pivotally lowered in the state where the distal end of the leg bottom section is set at the low position, the distal end of the leg bottom section is forcedly released from the mechanical engagement for the low position and shifted to the mechanical engagement for the high position.The present invention solves the abovementioned problem by a support mechanism, characterized in that one end of a knee bottom section 2 is supported on a bed frame 1 in such a manner that the knee bottom section can be pivotally raised and lowered, that one end of a leg bottom section 6 is pivotally rotatably connected with the other end of the knee bottom section, that a support stay 7 is pivotally rotatably connected with the underside of the other end of the leg bottom section, that a support means 11 of the support portion 9 provided at the opposite tip of the support stay is provided on the bed frame, that the support means consists of a guide portion 12 and an engaging portion 13 for movably supporting the support portion of the support stay, that said guide portion can support said support portion moved in relation with the pivotal rotation of the knee bottom section within the entire pivotal rotation range of the knee bottom section, and that the engaging portion is provided above or below the vacant portion adjacent to the end portion of the guide portion on which the support portion is located when the knee bottom section is pivotally lowered.
