To attain simplification in structure or cost reduction by reducing energy loss and heat generation by decreasing a current to be supplied to a solenoid as low as possible.

A solenoid supply current value iSOL is estimated from a detected battery voltage VBTT or an operating fluid temperature TMP. While using a voltage correction coefficient KV (calculated and set in S9) to be set smaller when the battery voltage VBTT is higher, for example, and a temperature correction coefficient KT (calculated and set in S8) to be set smaller when the operating fluid temperature TMP is lower, chopper ON time TCHP-ON (calculated and set in S10) during chopping operation is shortened and chopper OFF time TCHP-OFF (calculated and set in S11) is prolonged relatively so that the solenoid supply current value iSOL is decreased and the energy loss or heat generation can be reduced.