A variable-stiffness actuator, installed in a flexible member and capable of providing different stiffnesses for the flexible member, includes a shape-memory member that can transit in phase between first and second phases and an inducing member that causes phase transition between the first and second phases into the shape-memory member. The shape-memory member takes, in the first stare, a flexible state in which it is easily deformable by an external force, so as to provide lower stiffness for the flexible member, and, in the second stare, a rigid state in which it tends to take a memorized shape against an external force, so as to provide higher stiffness for the flexible member. The actuator further includes a stiffness calculator that calculates the stiffness of the actuator.
