PURPOSE: To improve the durability and reliability of a seal member and reduce the slide resistance of a spool by making the opening area of an orifice variable with the spool land of the spool at the side of an accumulator, and providing the seal member on a spool land at an opposite side.

CONSTITUTION: When a spool 120 is stroked with a stepping motor 180 in changing transmission torque characteristics, the opening area of an orifice 100 governing a flowing restriction effect is changed with a relative position between the spool 120 and the orifice 100. In the aforesaid transmission torque change control, the orifice 100 is opened and closed with the first spool land 123 formed with an annular groove 121 at the side of an accumulator chamber 90. Pressure in an oil passage continuous to the orifice 100, therefore, acts on the first spool land 123, and only pressure in the accumulator chamber 90 continuous to the side of the second spool via an oil passage within the spool 120 acts on the side of the second spool land 124 having seal members 126 and 127.