While a solenoid valve (23) of a gas passage (22) communicating with gas chambers (16, 17) is closed, an assist device (12) produces an assisting drive force by compression/expansion of a gas when a leg (3) flexes/stretches at a knee joint (specific joint) (8) and acts the force on the knee joint (8). While the solenoid valve (23) is open, no assisting drive force is produced. The solenoid valve (23) has a self-holding function. The solenoid valve (23) is so provided in the gas passage (22) that the difference between the pressures in the gas chambers (16, 17) acts so that the valve is closed during a predetermined period during which the solenoid valve (23) is closed. With this, the power consumption of the solenoid valve can be effectively reduced through a simple structure.