end a right end in fig1 by adopting this configuration although the space where the motor rotor 120 resides is allowed to communicate with the vacuum pump 20 the space where the motor stator 110 resides is isolated from the vacuum low pressure state fig2 is a perspective view intended to help a reader to understand the invention and shows some constituent components of the motor 100 specifically fig2 shows the pump main shaft 21 the sleeve 121 the balance rings 127 the washer member 122 and the bolt 124 as shown therein the balance rings 127 are fixed onto the sleeve 121 and the sleeve 121 is attached to the pump main shaft 21 then the bolt 124 is screwed into the pump main shaft 21 via the washer member 122 in the motor 100 of this embodiment although the rotor core 123 is placed on the outer circumferential surface of the sleeve 121 in this invention since the sleeve 121 is not an indispensable constituent element it is possible to omit the sleeve 121 next the function of the motor 100 configured as has been described heretofore will be described at the time of fabrication of a motor 100 with a sleeve 121 a rotor core 123 and end rings 125 assembled to a pump main shaft 21 a rotation imbalance is produced as.