A power transmission unit that can prevent unintentional disengagement of a clutch. A first set of teeth formed on an outer circumferential surface of a first rotary member is engaged with a third set of teeth formed on an inner circumferential surface of a second rotary member. A second set of teeth formed on an inner circumferential surface of the first rotary member is meshed with a fourth set of teeth formed on an outer circumferential surface of a third rotary member. A center of engagement between the first set of teeth and the third set of teeth is situated at a point withdrawn from a center of engagement between the second set of teeth and the fourth set of teeth in the direction to disengage the first set of teeth from the third set of teeth.