A driving speed Vd that is a speed of a driving force input point at which a driving force from a driving element is transmitted to an elastic member is acquired. An approximate rate Ve of change of an amount of elastic deformation Pe is computed based upon a plurality of values of the amount of elastic deformation Pe acquired as a quantized value at different points in time, by smoothing out an abrupt change in the values of the amount of elastic deformation Pe caused to appear by quantization. A deformation speed V is computed based upon the driving speed Vd and the approximate Ve by: V=A·(−Vd)+B·Ve where A is a coefficient increasing with increase in frequency of fluctuations in a position of the driving force input point, and B is a coefficient increasing with decrease in the frequency of fluctuations in the position of the driving force input point.