A power assist device for assisting a user to operate an object. The power assist device includes a pressure sensor that detects an operating force applied to the object by the user; a motor that outputs and applies a motive force to the object; and a controller. The controller records a history of the operating force detected by the pressure sensor; detects a periodically fluctuating component of the operating force based on the history of the operating force; calculates a component of a current operating force caused by motion of the user operating the object; calculates a correction value that excludes the component caused by motion of the user from the current operating force; calculates a power assist force based on the correction value; and outputs the power assist force as the motive force of the motor.