To provide a power transmission device which can cope with torque cutoff on gear change and run efficiently in running based on motor generator alone and has excellent mountability.

A power transmission device has an internal engine 11 and a power generator 2 as power sources and includes shift gears (31, 32, 44, 45 and 52) for shift stages consisting of pairs of gears composed, respectively, of an input axis 3 and first output axes 5 and of an input axis 3 and second output axes 5, two or more input axis common gears 33 and 34 being relatively rotatable with respect to the input axis 3 and always engaged in the power generator gear 221 to rotate synchronously, two or more first output axis common gears 42 and 43 being relatively rotatable with respect to the first output axis 4 and always engaged in the input axis common gear 33 and 34 and connecting means (35, 46, 47 and 53) of connecting individual gears rotating relatively with the input axis 3, the first output axis 4 or the second output axis 5 to individual axes in such a way as to be synchronously rotatable.