PURPOSE: To provide a speed reduction mechanism furnished with an overload preventing function and a reverse rotation preventive mechanism without providing a special accessorial mechanism.

CONSTITUTION: This 18 a speed reduction mechanism applied to a gear motor used for a lifter of business equipment, etc. A planetary gear structure of a trocoidal tooth form inscribing engagement type is adopted as a basic speed reduction mechanism, an external tooth gear 32 and an internal tooth gear 33 out of each of elements of this speed reduction mechanism are formed of resin, rigidity of which is small, and an output shaft and a bearing to support the external tooth gear 32 are made of metal, rigidity of which is large. Consequently, it becomes possible to mechanically cause lowering of efficiency by maintaining rigidity and rotational smoothness at the time of normal driving and increasing engagement loss by deformation of the external tooth gear 32 and the internal tooth gear 33 against torque more than required by an objective member to be driven. Additionally, it is possible to correspond to a reversing load from the side of the output shaft 36 with the same engagement loss.