Abstract: It is possible to quickly and efficiently support a user with a limited lower limb motor function in a posture transition with a simplified configuration. A support mechanism that supports a posture transition includes a first link that does not change a posture, a second link that is rotatably coupled to the first link, and an actuator configured to rotate the second link with respect to the first link, in which, when a load on a connection part of the second link and the first link changes due to a change in an upper body posture of a user, based on a relative positional relationship between an ankle joint and a knee joint of the user in a direction orthogonal to a vertical direction in a sagittal plane of the user, the actuator drives the second link while changing a support moment on the user according to a relative positional relationship between the knee joint and an upper body centroid of the user, or a knee joint angle.