A power transmission mechanism for transmitting driving power between a shaft and a rotor connected to the shaft includes a cylindrical adapter located between the shaft and the rotor and a screw seat arranged on the shaft. The adapter has a first internally threaded portion in its inner peripheral surface and a first externally threaded portion in its outer peripheral surface. The first internally threaded portion is engaged with a second externally threaded portion of the shaft, and the first externally threaded portion is engaged with a second internally threaded portion of the rotor. The screw seat has a screw seat surface. The adapter is screwed onto the shaft such that the adapter is pressed against the screw seat surface, and the rotor is screwed onto the adapter such that the rotor is pressed against the screw seat surface.