To prevent abrasion and damage in a cam part and a rotary cam part by arranging a bypass groove, which communicates one chamber with the other chamber until the rotary cam part locked by means of the cam part is released, in the inner circumference of a housing.

A bypass groove 3d, which communicates with chambers (a), (b) together until a rotating directional load applied to a rotary cam body 51 from a valve seat slightly exceeds a slant in a second cam face 15b via resistance of a coil spring 41 so as to reach a third cam face 15c, is arranged in the inner circumference of a housing main body 2, so that initial braking force rotating the rotary cam part 53 in the braking force applying direction can be reduced extremely. Therefore, no large load is applied when locking of the rotary cam part 53 is released, so that mutual scraping between a projection part, which is formed by the second cam face 15b and the third cam face 15c, and the rotary cam 53 is prevented, and consequently, abrasion and damage in both of the cam parts 14, 53 can be prevented.