A housing is adapted to be placed on a floor and is provided on its top face with a left foot support 2a and a right foot supports 2b respectively bearing left and right feet of a user. A drive unit 3 is provided to displace the left and right foot supports 2a and 2b in a mutually linked manner. The drive unit 3 is configured to reciprocate said left and right foot supports (2a, 2b) to move a representative point (gl, g2, g3, d1, d2, d3) of each supports (2a, 2b) in a forward/rearward direction, or to reciprocate said left and right foot supports (2a, 2b) in such a manner as to vary a lateral distance between said left and right foot supports (2a, 2b) with regard to representative points (gl, g2, g3, d1, d2, d3) of the left and right foot supports (2a, 2b), while allowing each of said left and right foot supports (2a, 2b) to pivot about an axis along its lateral direction, its upright axis perpendicular to a top surface of the corresponding one of said left and right foot supports (2a, 2b), or an axis along its forward/reaward direction.
