To reduce idling noise excellently without affecting combustion by decreasing an actual compression ratio in an idling state.

This control unit is provided with an intake throttle valve 3 for decreasing an actual compression ratio in an idling running state, a shallow piston cavity with a large diameter but no reduction of area, a fuel injection valve disposed in a combustion chamber substantially opposing the center of the cavity, a controller 20 performing two stages of fuel injection; pilot injection and main injection from the fuel injection valve in an idling running state. The pilot injection is performed in an idling state while the main injection is performed with raised ambient combustion temperature. Furthermore, adhesion of fuel to a wall surface inside the cavity is prevented in order to avoid misfire and increase of unburned HC.