PURPOSE: To activate injected gas by high temperature and high pressure gas by providing a control valve in a connection port between a subsidiary chamber and a main chamber of a methanol engine and shutting the control valve in the middle of the expansion stroke during a partial load in the engine so as to leave a combustion gas partially.

CONSTITUTION: The outer circumference of a subsidiary chamber 3 is surrounded by an air layer 32 so as to be insulated from the heat and to keep the high temperature, and a control valve 4, which is opened or closed by means of excitation of an electromagnet 5, is arranged between the subsidiary chamber 3 and a main chamber 12. During a partial load in the engine, the control valve 4 is closed in the middle of the expansion stroke so as to maintain a combustion gas, and injected methanol is activated by means of the high temperature and high pressure combustion gas, and then, injected into the main chamber 12 by opening the control valve 4 so as to be bunt sufficiently, so that the combustion during a partial load is improved.