A control unit 20 includes an initial current value calculation unit 22 for calculating an initial current value when a caliper pressure shifts to a pressure increasing state, a target current value setting unit 23 which, when no pressure increasing cycle was performed last time, estimates a road surface friction coefficient, determines a target differential pressure based on the road surface friction coefficient so estimated, and sets a current value corresponding to this target differential pressure as a target current value, and a valve opening amount adjusting unit 25 which reduces an energization amount from the initial current value towards the target current value with a predetermined gradient.