A starter for an engine is equipped with a planetary gear speed reducer, a restraining mechanism, and a shock absorber which is made up of a rotary disc, a stationary disc, a case, and a load applying mechanism. The load applying mechanism applies a load to the stationary and rotary discs to develop a given degree of friction therebetween. When an excessive torque acts on the rotary disc through the internal gear, the shock absorber induces the rotary disc to rotate against the given degree of friction to absorb the torque to eliminate a physical impact to be exerted on the planetary gear speed reducer. The restraining mechanism restrains the load applying mechanism from moving in a radius direction of the internal thread to avoid undesirable engagement of the load applying mechanism with the internal thread, thereby ensuring the stability in applying the load to the stationary and rotary discs.