A motor assembling method for an electric motor including a first housing having an opening disposed on one side in an axial direction, a rotary shaft having a rotor, and located along the axial direction in a center portion inside the first housing, a stator located around the rotor inside the first housing, a second housing fixed to the opening, and a first bearing and a second bearing which are disposed in the first housing and the second housing, respectively, and which rotatably support each end portion of the rotary shaft in the axial direction, the method comprising: a step of inserting the stator into the first housing through the opening; a step of fitting a shaft portion of a centering jig to an inner peripheral surface of the stator and a centering portion of the centering jig, disposed in a tip portion of the shaft portion to the mounting portion of the first bearing in the first housing, and fitting a flange portion of the centering jig to the opening; a step of fixing the stator on which centering is performed using the centering jig to the first housing; a step of removing the centering jig from the first housing; and a step of inserting the rotary shaft, the first bearing, and the second bearing into the first housing, and fixing the second housing to the opening, wherein after the centering jig is inserted into the first housing, the stator is pressed against the first housing in the axial direction by a pressing member of the centering jig, and the stator is fixed to the first housing in this state. a first housing having an opening disposed on one side in an axial direction, a rotary shaft having a rotor, and located along the axial direction in a center portion inside the first housing, a stator located around the rotor inside the first housing, a second housing fixed to the opening, and a first bearing and a second bearing which are disposed in the first housing and the second housing, respectively, and which rotatably support each end portion of the rotary shaft in the axial direction, the method comprising: a step of inserting the stator into the first housing through the opening; a step of fitting a shaft portion of a centering jig to an inner peripheral surface of the stator and a centering portion of the centering jig, disposed in a tip portion of the shaft portion to the mounting portion of the first bearing in the first housing, and fitting a flange portion of the centering jig to the opening; a step of fixing the stator on which centering is performed using the centering jig to the first housing; a step of removing the centering jig from the first housing; and a step of inserting the rotary shaft, the first bearing, and the second bearing into the first housing, and fixing the second housing to the opening, wherein after the centering jig is inserted into the first housing, the stator is pressed against the first housing in the axial direction by a pressing member of the centering jig, and the stator is fixed to the first housing in this state. a step of inserting the stator into the first housing through the opening; a step of fitting a shaft portion of a centering jig to an inner peripheral surface of the stator and a centering portion of the centering jig, disposed in a tip portion of the shaft portion to the mounting portion of the first bearing in the first housing, and fitting a flange portion of the centering jig to the opening; a step of fixing the stator on which centering is performed using the centering jig to the first housing; a step of removing the centering jig from the first housing; and a step of inserting the rotary shaft, the first bearing, and the second bearing into the first housing, and fixing the second housing to the opening, wherein after the centering jig is inserted into the first housing, the stator is pressed against the first housing in the axial direction by a pressing member of the centering jig, and the stator is fixed to the first housing in this state.