A starter motor is disclosed which includes: 1) a hollow yoke having an open end; 2) a magnetic field system provided on an inner periphery of the yoke; 3) an armature surrounded by the magnetic field system with a predetermined gap therebetween, the armature having an armature shaft; 4) an end frame that closes the open end of the yoke and has a bearing box formed therein, the bearing box being recessed inward of the yoke from an end face of the end frame and having an open end on the end face of the end frame, the bearing box also having a bearing disposed therein to rotatably support an end portion of the armature shaft; and 5) a flexible seal plate that is attached on the end face of the end frame to hermetically seal the open end of the bearing box formed in the end frame.