An irreversible mechanism is provided with an input shaft that receives input of rotational drive force, an output shaft that receives transmission of torque from the input shaft, a ball ramp part that generates radial force when load torque is applied to the output shaft, a brake lining that receives the radial force from the ball ramp part, a brake drum that the brake lining is pressed against when the brake lining receives the radial force from the ball ramp part, and a housing that accommodates the input shaft, the output shaft, the ball ramp part, the brake lining and the brake drum inside.