A variable-stiffness actuator includes a shape-memory member that increases in stiffness on heating, a heating member arranged to surround the shape-memory member along a longitudinal axis of the shape-memory member, and a heat transmitting medium arranged to surround the heating member along a longitudinal axis of the heating member. The heating member generates heat in response to supply of a current, so as to heat the shape-memory member. The heat transmitting medium is deformed to decrease an inner diameter of the heat transmitting medium to come into contact with the heating member, so as to cool the heating member. The heat transmitting medium is deformed to increase the inner diameter to come out of contact with the heating member.
