Provided is a step assist device including a first phase angle modifying section that, according to repetition of the step movement, modifies in a staged manner the phase angle input thereto, based on a predetermined phase change pattern, such that a phase pattern of the phase angle approaches a straight line relative to the passage of time, and outputs the modified phase angle as a first modified phase angle. The control section acquires a target value based on a continuous change pattern of the auxiliary force set in advance for at least one of the first modified phase angle and a prescribed phase angle, and controls the providing section according to the target value.
