A support mechanism in which 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. Also, one end of a leg bottom section (6) is pivotally rotatably connected with the other end of the knee bottom section, a support stay (7) is pivotally rotatably connected with the underside of the other end of the leg bottom section, and a support device (11) of a support portion (9) provided at the opposite tip of the support stay is provided on the bed frame. The support device includes a guide portion (12) and an engaging portion (13) for movably supporting the support portion of the support stay. The guide portion can support the 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. The engaging portion is provided above or below a 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.
