It is possible to provide a power assist device which can maintain a stable contact state without causing an oscillation phenomenon even if a robot is brought into contact with an environment. A method for controlling the power assist device is also provided. The power assist device includes: an inner force sensor which detects an operation force applied by an operator; an operation handle having the inner force sensor; a robot arm which supports the operation handle; an actuator which drives the robot arm; the actuator and a control device which measure or estimate a force applied when the robot arm is brought into contact with an environment; and the actuator and the control device which detect or estimate a motion speed of the operation handle. The control device acquires a corrected external force according to the operation force detected by the inner force sensor and an external force detected by the actuator and the control device as external force derivation means and controls the actuator so that the corrected external force acts on the operation handle.