A muscular-force supplementing device of the present invention includes an artificial muscular-force generating section and a control section that controls the driving of the artificial muscular-force generating section. The artificial muscular-force generating section includes a pair of mounting portions worn at two positions on both sides of a wrist joint of the user, and an actuator hinged on connecting portions of the mounting portions. The actuator is a device having therein a plurality of hydraulic cylinders to be operated in a dual-stroke manner. When hydraulic oil serving as the fluid is supplied to pressure chambers, a pair of piston rods are extended, and linear actuator force is transmitted to the connecting portions of the mounting portions, thereby transmitting bending force to the joint of the user.