PURPOSE: To prevent leak of fluid past a port part, by a method wherein a pintle part formed in a trapezoid in cross section, in which fluid feed and discharge ports extending to a cylinder are formed, is foreced into press contact with a recess formed in a trapezoid in cross section, to which flow passages leading to the ports, are open, by means of a magnetic force.

CONSTITUTION: A pintle part 14a, formed in a trapezoid in cross section, in which fluid feed and discharge ports 29 and 39, extended to a cylinder 13, are provided, is engaged with a recess part, similarily formed in a trapezoid in cross section, to which flow passages 25 and 35, communicated with the ports, are open. A magnet is attached on the casing side opposite to a main shaft facing the pintle part 14, and attracts the pintle part 14 rightward. This causes a contact surface part 14b between the ports 29, 39 and the flow passages 25, 35 to be forced into contact with no gap, resulting in prevention of leak of fluid past said part.