An electric motor for driving a piston pump assembled thereto is disclosed wherein a load from the piston pump acts radially on an armature shaft of the electric motor. The electric motor is assembled such that the outer ring of a yoke bearing is connected to a yoke, and the outer ring of a first pump housing bearing is press fit to a pump housing. One end of the armature shaft is press fit to the yoke bearing inner ring, and the other end of the armature shaft is press fit to the first pump housing bearing inner ring. Thus, wobbling of the armature shaft in the axial and radial directions is minimized or eliminated.