A power transmission mechanism that permits power to be transmitted from an external driving source to a rotary shaft of a rotating machine includes a first rotating body, a second rotating body, an armature, and a spring clutch. The spring clutch is wound about an outer circumference of the first rotating body and an outer circumference thereof is surrounded by the extended portion of the second rotating body. The spring clutch has a first end connected to the first rotating body, and the second end is connected to the armature. Supply of current to the electromagnetic coil causes the armature to adhere to the second rotating body. When the armature rotates, the diameter of the spring clutch is increased such that the spring clutch is pressed against the inner circumferential surface of the extended portion and the first rotating body and the second rotating body are coupled to each other.