To efficiently and inexpensively put an electric vehicle into practice.

Principle of pascal is adapted to power of an electric machine 3 by supplying fluid 6 between plural pistons 4 and 5. Driving force of an engine is thus kept or increased, so that batteries 1, 2 are downsized. The batteries 1, 2 are mounted on one vehicle at the same time. The batteries 1, 2 are sequentially used similarly to a cassette type. Successive operation is thus enabled and suspension for electric charging is avoided. It is thus possible to put an electric vehicle into practice inexpensively and efficiently.