A control device of a variable stiffness mechanism controls a total stiffness by variably outputting a voltage from a DC power source 10 to an “m” number of stiffness control elastomers 1 among a plurality of dielectric elastomers 1 of a variable stiffness mechanism 100. Further, in a state in which a square-wave signal is being output from a square-wave power source 11 to an “n” number of electrostatic capacitance estimation elastomers 1, a detection value of energizing current is acquired and the acquired detection value is used to estimate an electrostatic capacitance.