This irreversible mechanism (1) includes a rotatable input shaft (3), an output shaft (4) that rotates according to the input shaft, a conversion mechanism (5) that converts back drive torque from an output shaft side to an axial force, a brake portion (6) that includes a first brake surface (61), and a second brake surface (56) provided to rotate integrally with the output shaft and pressed against the first brake surface according to the axial force of the conversion mechanism. The first brake surface and the second brake surface include taper surfaces tapered in the action direction of the axial force of the conversion mechanism.