In order to increase (increase the amount of correction) points where the rotational balance of a rotating shaft of a supercharger built into an electric motor is adjusted so as to maintain a sufficient motor driving force, even under high temperature conditions, a balance-adjusting structure for the supercharger built into the electric motor is characterized in that a rotor core (5), formed by laminating magnetic steel plates (51), is interposed between rotor blades disposed on one end or both ends, and a rotational balance-adjusting member made of iron (magnetic induction member), which is thicker than the thickness of the laminated magnetic steel plates (51), is disposed on an end of the magnetic steel plates (51) or between the laminated layers.