PURPOSE: To increase the compression ratio and swirl ratio of a direct injection type diesel engine at the time of low speed, and reduce respective ratios at the time of high speed.

CONSTITUTION: A cylinder groove 22 is provided in the center area of the cavity 21 of a piston 20 to insert therein a variable piston 30 actuated by oil pressure and energized downward by a spring 33. A check valve 50 is provided in a circuit 44 for leading pressure oil into a pressure chamber 34, and relief valves 60, 65 are provided respectively in exhaust circuits 25, 27 for communicating the pressure chamber 34 to the out side. The circuits 25, 26 are closed by the relief valves 60, 65 at the time of low speed, and the variable piston 30 rises by oil pressure to increase compression ratio and swirl ratio. The spool 61 of the relief valve 60 rises near a piston top dead point at the time of high speed by inertia force so as to open the circuit 25. On the other hand the spool 61 of the relief valve 65 falls down near a piston bottom dead point, to open the circuit 27. Oil pressure in the pressure chamber 34 is reduced, and the variable piston 30 falls down to reduce the compression ratio and the swirl ratio.