A circulation pressure is supplied to a start device (2), and a linear solenoid valve (10) regulates the operating pressure which engages and disengages a lockup clutch (3) by means of the difference between the operating pressure and the circulation pressure. A spool section (121) of the linear solenoid valve (101) is provided with a first feedback oil chamber (12b) which feeds back and applies the operating pressure to a spool (12p), and also with a second feedback oil chamber (12a) which feeds back and applies the circulation pressure to the spool (12p) in the direction opposite to the direction in which the first feedback oil chamber (12b) feeds back and applies the operating pressure. The pressure receiving area (A1-A2) of the spool (12p) in the first feedback oil chamber (12b) and the pressure receiving area (A1-A2) of the spool (12p) in the second feedback oil chamber (12a) are set to be the same. The configuration enables, even if the circulation pressure varies, the hydraulic operational force by the operating pressure in the first feedback oil chamber and that of the circulation pressure in the second feedback oil chamber to be equal, and as a result, the engagement/disengagement control or the slip control of the clutch can be accurately performed.