A power transmission mechanism is attached to a housing of a rotary machine for inputting power to a rotary shaft of the rotary machine and includes a rotary body that is rotatably supported by the housing through a rotary body bearing and an electric motor, a stator of which is supported by the housing through a stator bracket. A positioning structure for the power transmission mechanism includes a bearing positioning means and a bracket positioning means. The bearing positioning means positions the rotary body bearing forward and rearward in an axial direction of the rotary shaft relative to the housing and has front and rear positioning surfaces. The bracket positioning means positions the stator bracket forward and rearward in the axial direction and has front and rear positioning surfaces. At least one of the front and rear positioning surfaces of the respective positioning means is shared with each other.