A brush DC motor with reduction mechanism includes: a brush DC motor including a motor case, a stator section and a rotor section having a substantially cylindrical core, the stator section and the rotor section being received in the motor case; and a first reducing section drivable by the motor. The first reducing section includes a first sun gear, an external ring gear positioned around the sun gear and several planetary gears held between the external ring gear and the sun gear and engaged with the sun gear and the external ring gear. The first reducing section further includes a planetary carrier for locating the planetary gears in their true positions. Multiple parallel linear splines are formed on a circumference of the core at intervals. One end of the core with the splines formed on the circumference of the end serves as the sun gear of the first reducing section.