To determine the pressure difference between first and second conduits easily providing a solenoid valve to be coupled with a conduit and determining the pressure difference based on a predetermined pressure gradient, a signal characterizing the duration of control of a first solenoid valve, and the pressure difference before control.

An inlet valve 100 is coupled through a first port and a first conduit 105 with a main brake cylinder 110 and coupled through a second port and a second conduit 115 with a wheel brake 120. The conduit 105 is normally applied with a pressure PV while the conduit 115 is applied with a pressure PH for determining the brake power of wheel. A connection passage extends from the conduit 115 to the first port of an outlet valve 160 having a second port coupled with a return carrying pump 170. The valves 100, 160 are so-called 2/2 solenoid valves. Pressure difference between the pressures PV and PH is determined based on a predetermined pressure gradient, a signal characterizing the duration of control of a first solenoid valve, and the pressure difference before control.