An assist device includes an upper-body belt attached to the upper body of a user, first and second belts attached to the knees, a first wire coupling the upper-body belt to the first belt, a second wire crossing the first wire, a third wire coupling the upper-body belt to the second belt, a fourth wire crossing the third wire, and a motor coupled to one end of each of the first to fourth wires. When assisting users with walking, tensions equal to a first threshold value or greater are applied to one of the first and second wires and one of the third and fourth wires by the motor at different times. When detecting slacking of the upper-body belt, tensions equal to the first threshold value or greater are simultaneously applied to one of the first and second wires and one of the third and fourth wires by the motor.