A main-bearing outer diameter of an electric motor is made smaller than a rotor outer diameter without causing a drop in production efficiency, and the ease of performing a shrink-fitting step is also enhanced by making it possible to mount a rotor to a main shaft in an early stage. A bearing journal portion (14b) of a main shaft (14) of an electric motor is formed in a shape into which an inner race (18b) of a main bearing (18) can be press-fitted from both sides in the axial direction thereof, and, in addition, an outer diameter of the bearing journal portion (14b) is made larger than an outer diameter of a rotor press-fitting portion (14a), whereas a bearing bore portion (5a) of a partition member (5) is formed in a shape into which an outer race (18a) can be press-fitted from both sides in the axial direction thereof. It is preferable that a space (S) into which a press-fitting tool for press-fitting the main bearing (18) can be inserted be provided between the rotor press-fitting portion (14a) of the main shaft (14) and the bearing journal portion (14b) thereof.