To provide an energy conversion system for extremely reducing an energy loss.

Cylinders are continued by three pieces, and the center is formed as an operation chamber cylinder, and both sides are formed as auxiliary cylinders with same diameter different from that of the operation chamber cylinder. The stroke volume ratio of the auxiliary cylinder and the operation chamber cylinder is set to the adiabatic contraction ratio or the adiabatic expansion ratio. A selector valve and a passage of a working fluid are attached by arranging a partition plate on both sides of the auxiliary cylinder, and a heater-cooler heat recovery unit is connected to an external part. The valve switches the passage by opening and closing by being alternately pushed at the top dead center or the bottom dead center of a piston. Six chambers can be formed of three cylinder and three pistons, and three operation areas are made by respectively connecting these six chambers by two pieces, and three strokes of an adiabatic volume change, a fluid constant pressure heating change and a fluid constant volume cooling change, are simultaneously performed.