A motor with speed reduction mechanism, comprising: a cylindrical yoke; a magnet arrangement consisting of four magnets, said magnet arrangement having four poles, two different magnetic poles being alternately disposed on an inner peripheral surface of the yoke; an armature including: an armature shaft; a commutator fixed to the armature shaft and formed of a plurality of segment parts; and an armature core fixed to the armature shaft and around which a winding wire is wound, the armature being rotatably disposed on the inner side of the magnet; a brush holder unit including first and second brushes slidably contacting with the commutator and disposed so as to be shifted by approximately 90 degrees from each other with respect to a center of rotation of the armature shaft and brush holders holding the first and second brushes; and a metallic gear housing connected to the yoke, and including a speed reduction mechanism accommodating part having accommodated therein a speed reduction mechanism engaged with a worm formed on the armature shaft; a control board for controlling rotation of the armature; and a gear housing cover on which the control board is mounted, the gear housing cover covering an opening of the gear housing, characterized in that: the first and second brushes are inclined with respect to each other at an angle of substantially 90 degrees in a circumferential direction of the armature shaft, the brush holder unit is accommodated in the gear housing, a first heat dissipating member is formed on the gear housing in the vicinity of the first and second brushes. a cylindrical yoke; a magnet arrangement consisting of four magnets, said magnet arrangement having four poles, two different magnetic poles being alternately disposed on an inner peripheral surface of the yoke; an armature including: an armature shaft; a commutator fixed to the armature shaft and formed of a plurality of segment parts; and an armature core fixed to the armature shaft and around which a winding wire is wound, the armature being rotatably disposed on the inner side of the magnet; a brush holder unit including first and second brushes slidably contacting with the commutator and disposed so as to be shifted by approximately 90 degrees from each other with respect to a center of rotation of the armature shaft and brush holders holding the first and second brushes; and a metallic gear housing connected to the yoke, and including a speed reduction mechanism accommodating part having accommodated therein a speed reduction mechanism engaged with a worm formed on the armature shaft; a control board for controlling rotation of the armature; and a gear housing cover on which the control board is mounted, the gear housing cover covering an opening of the gear housing, characterized in that: the first and second brushes are inclined with respect to each other at an angle of substantially 90 degrees in a circumferential direction of the armature shaft, the brush holder unit is accommodated in the gear housing, a first heat dissipating member is formed on the gear housing in the vicinity of the first and second brushes. the first and second brushes are inclined with respect to each other at an angle of substantially 90 degrees in a circumferential direction of the armature shaft, the brush holder unit is accommodated in the gear housing, a first heat dissipating member is formed on the gear housing in the vicinity of the first and second brushes.