An automatic transmission consisting of a planetary gear unit comprising in combination an input shaft (1 ), an input gear (2), a key planet gear (3), an output gear (4), an output shaft (5), a bearing planet gear (6), a rotating frame (7), a rotor (8), wherein the planet shaft is the rotor (8) having a certain inertia moment, so its revolving is equivalent to the precession of a gyroscope due to an external force wherein the precession-production external force is the load acting on the output shaft (5). Hence the revolutionary speed of the rotor (8) varies depending upon changes in the output load and the same holds true for the rotational speed of the output shaft (5), which makes the speed ratio spontaneously vary with changes in the outpot load. Accordingly, the transmission of the present invention operates at speed ratios spontaneously varying with changes in load, without any controlling operation.