To eliminate a play angle even when a direction of a load is changed in a lacked state while maintaining the locked state in both of forwarding and backwarding directionsand having a new function free to simultaneously rotate in the both forwarding and backwarding directions by operation.

Holders 5, 6 to be overlapped inside and outside are provided between an inner ring 1 and an outer ring 2 having cam surfaces 9. Two engaging elements 3 and elastic bodies 17 to press on the cam surfaces 9 by extending intervals of the engagement elements 3 are provided in pocket communicating parts 14 of the holders 5, 6. Unlocking means 7 to narrow the pocket communicating parts 14 by deviating the inside and outside holders 5, 6 in the rotating direction are provided. The unlocking means 7 is constituted of an operating member 21 to press the outside holder 6 in the axial direction and a guide projected part 11 of the inner ring 1 to slightly rotate the holders 5, 6 in association with such a pressing motion, etc.