In a small starter motor which has a substantially closed space therein, it is necessary to blow out brush dust particles therefrom. On a coil end surface or both coil end surfaces of an armature, a fan generating an air circulation is formed utilizing gaps formed between individual coil ends on the surface without adding any other parts. Also, a side wall is disposed next to the coil end surface in parallel thereto which closes an axial end of a yoke in which the armature is coaxially disposed. In the space formed between the side wall and the coil end surface, an air circulation is generated when the armature rotates. The side wall has an air inlet hole and the yoke has an air outlet hole. Air coming into the space from the air inlet hole is blown out from the air outlet hole by the circulation generated by the fan made on the coil end surface. The brush dust particles are blown out by the circulated air, and the coil ends are cooled off at the same time. A distance between the side wall and the coil end surface and a depth of the gaps formed between the individual coil ends have to be optimized to obtain a good air circulation.