A driving apparatus of an electric vehicle that enables secure positioning and easy assembling, and a method for assembling the driving apparatus of the electric vehicle. A driving apparatus of an electric vehicle includes an electric motor provided in an electric motor case, the electric motor having an output shaft protruding outside the electric motor case, a planetary gear mechanism provided in a speed reducer case, the planetary gear mechanism transferring the driving force at a reduced speed from the output shaft to the driving wheel. The output shaft integrally includes a sun gear meshed with a planetary gear of the planetary gear mechanism. The speed reducer case includes a positioning section disposed concentrically with the output shaft, the positioning section being combined with the electric motor case. A meshing length between the sun gear and the planetary gear is larger than an insertion depth of the positioning section.