use of plastic with a low coefficient of friction gives still more decrease in noise and increase in efficiency further there is little backlash because it is possible to achieve a high reduction ratio by assembling a small number of gears and the miniature reduction gear costs less because of the small number of parts still further the reduction mechanism has little noise because lower case 6 and upper case 26 are closed tightly giving a miniature reduction gear that resists entry of dust fig5 shows an example that makes the construction of the present invention still more effective it has sliding mechanism or one way clutch mechanism 51 between lower case 6 and fixed inner gear 10 the planetary gear reduction mechanism of the present invention creates a differential between the fixed inner gear and the movable inner gear by the solar rotation of the planetary gears and the movable inner gear rotates relative to the stationary fixed inner gear when a sliding mechanism is added between lower case 6 and fixed inner gear 10 and an overload is applied to output shaft 23 the sliding mechanism starts to slide at above a certain torque priorly although reduction mechanisms for overload were used in miniature reduction gear and sliding mechanisms were used to prevent damage in other parts with the present invention the sliding mechanism is added with almost no change in the outer shape.