According to a walking motion assist device 10, an assisting force T of a leg in a standing-leg period is controlled to indicate a greatest value or a maximum value in a first designated period (refer to the one-dot chain line and the two-dot chain line in t=t1 to t2). By doing so, in the first designated period (an initial period or a prior period) of one leg, a translation of a body derived from a floor reaction force acting on the one leg is largely prompted compared to other periods of the standing-leg period. A smooth starting of floor leaving and bending motion of the other leg is prompted so as to follow the translation of the body.