To effectively prevent entrance of dust to the internal part of a body housing.

In this seal mechanism, a packing 18 made of rubber is contained in the whole periphery of a joint part between a gear housing 3 to form a body housing 2 and a gear housing cover 4 and the joint is sealed. A V-shaped valve element 19 is extended to the packing 18 and a protrusion 20 on the tip end thereof makes contact with an air hole with the protrusion closing an air hole 21 to intercommunicate the front and the rear of a partition wall 3a. Meanwhile, a seal retainer 22 and a dust seal are arranged at the through-hole of the slider 12 of a gear housing cover 4 to perform lubrication and seal of the through-part of the slider 12.