Power assist control is performed to deal with feeling or sensitivity with which a person applies a force. An auxiliary force F or a speed F is outputted based on an assist object force X detected by a force sensor (114) for detecting a force acting on an object. Relation between the X and the auxiliary force F or the speed F is represented by F indirect proportion to f(X) based on the function f(X) where the assist object force X is a variable. The f(X) is an increasing function and a nonlinear function having a derivative f’(X) of decreasing function. The f(X) is a (1/n)-order function or a logarithmic function. The function fup(X) when the assist object force increases is different from the function fdown(X) when the assist object force decreases.