Appropriate reaction force imparting type work assist device

The present invention relates to a device (B) for assisting an operator (P) in carrying heavy articles (W). The device (B) is provided with a movable body (2) for supporting the heavy article (W), an actuator for actuating the movable body (2), and a controller for adjusting the output of the actuator. The controller adjusts the actuator such that a force obtained by subtracting a resultant force of force proportional to acceleration of the heavy article, force proportional to speed, and force proportional to position value from force required for moving the heavy article (W) with a same movement as that performed at that point. Further, coefficients proportional to acceleration, speed, and position value are changed during the series of carrying work. Especially, it is useful that the coefficients be changed according to the task process phases. It makes an operator (P) feel as if he were changing assist device to an appropriate one for each task process phase. The operator (P) is able to work under various environments with a feeling which is convenient for performing his work well.