A series of geared motors in which change gear ratios of 1/2-1/100 or a wider range of change gear ratios are realized in the single series based on the unified design concepts of low noise, high rigidity and high durability, and in which the separation, exchange, etc. of the individual speed change gears and motors proper of the geared motors are facilitated. In the series, a speed change mechanism in each of the geared motors constituting a group of "high" change gear ratios comprises a single-stage type planetary gear mechanism of oscillating inner gearing system, a group of "medium" change gear ratios comprises a single-stage type simple planetary gear mechanism, and a group of "low" change gear ratios comprises a modified simple planetary gear mechanism including a sun gear (302 in FIG. 1), a first planet gear (304A), a second planet gear (304B) which rotates unitarily with the first planet gear (304A) and which has a larger number of teeth than the first planet gear (304A), and an internal gear (306). Herein, the principal gear ratio part of each of the speed change gears of the geared motors constituting the respective groups of change gear ratios is sealably supported at both the ends thereof.