A control unit that controls control currents supplied to shock absorbers that generate damping forces according to the control currents determines coefficients of two functions that functionally identify an equivalent damping coefficient and an equivalent spring constant of each suspension based on relationships between the control current supplied to each shock absorber and the coefficients of the two functions that change according to the control current and a frequency of a relative vibration between a sprung and an unsprung of a vehicle, and calculates a relative displacement or a relative velocity between the sprung and the unsprung based on a vertical acceleration detected by a detection device and the two functions in which the coefficients are determined.