PURPOSE: To improve fuel consumption in time of a drive of low speed and low load and to alleviate the limit of the maximum output due to mechanical loss in time of a drive of high speed and high load by a method wherein combustion chambers in some of cylinders are of the output priority type and those in the others are of lamellar combustion type.

CONSTITUTION: There are provided a cylinder group A comprising cylinders 4, 10 having a output priority type combustion chamber structure and a cylinder group B comprising cylinders 6, 8 composed of a main combustion chamber 172 provided with a piston 24 and a subchamber 176 connecting to the combustion chamber 172 through a jet hole 174. Besides, structurally, concentrated mixed gas is supplied from a concentrated mixed gas preparing means 212 of a carburetor 15 to the subchamber 176 and dilute mixed gas is supplied to the chamber 172. In the above-said constitution cylinder group A stops and cylinder group B only operates in time of a low speed and load drive while both cylinder groups A, B operate in time of a high speed and load drive. In this case, concentrated and dilute lamellar combustion is carried out in the whole combustion chamber 170.