To suppress noise generated in operation.

First gears 24, 27 and second gears 25, 28 are rotatably installed on two actuation chambers 11, 12 provided on a housing 10, and the first gear and the second gear in each actuation chamber 11, 12 are installed in such a way that their tooth phases are mutually deflected by 1/4 pitch, and that teeth of one gear of two gears in each engaged gear set are simultaneously engaged with flanks of two adjacent teeth of the other gear along two parallel contact lines.