An exercise aid device has a hip supporting member movable relative to a base, footrests movable relative to the base, drive means for driving the hip supporting member, body constitution estimating unit for estimating at least one of fat mass and muscle mass of a user, and a controller for the drive means. The controller controls the drive means such that a load acting on a femoral region by own weight of the user supported on the hip supporting member changes according to a relative positional displacement between the user's toe and trochanter major, the positional displacement is allowed in a direction of flexion and extension of knee joint of the user, and an angle of the knee joint is maintained substantially constant. In addition, since the controller controls the drive means by use of an output of the body constitution estimating unit, it is possible to provide an exercise with less burden to the knee joint and a suitable strength for the user.