To stop a combustion piston and a pumping piston at such a position as to provide the combustion piston with a proper top dead center position.

The device comprises at least one duel piston assembly. When the pumping pistons 15 and 16 move from a bottom dead center to a top dead center, fluid is sucked at low pressure through a low pressure fluid suction valve into a fluid pressure cylinder 12. When the pumping pistons move from the top dead center to the bottom dead center, fluid is discharged at high pressure that is higher than the low pressure. A position indicator on the duel piston assembly is read, and a position signal is generated. Based on the position signal, an electronic control unit determines a stop position of the duel piston assembly with which a target compression ratio is provided. The electronic control unit generates an instruction signal for closing the low pressure fluid suction valve at a constant stroke so as to stop the duel piston assembly at the instructed stop position. The target compression ratio is thus achieved by extracting force of fluid pressure.