To obtain a self-lock mechanism having high reliability, which can stably carry out a drive operation and a stop operation.

A self-lock mechanism includes a screw member 2 driven to rotate, a first nut member N1 and a second nut member N2 screwed to the screw member 2, an urging member 3 that rotates the first nut member N1 and the second nut member N2 relative to each other, and a nut holder 4 rotatably holding the first nut member N1 and the second nut member N2. The nut holder 4 includes: a first receiving part U1 that regulates axial movement of the first nut member N1 urged to rotate by the urging member 3; and a second receiving part U2 that regulates axial movement of the second nut member N2 urged to rotate by the urging member 3.