To efficiently perform self-ignition of a four-stroke engine during a partial load.

A four-stroke internal combustion engine comprises one or more cylinders 1 which have one or more first intake means 2 and one or more first exhaust means 3. During a partial load, this four-stroke internal combustion engine performs self-ignition combustion by such a method that an external volume 10, which is provided independent from the cylinder 1 and is connected to the cylinder 1 with both a special port and a special pipeline 9, is filled with a combustion gas while at least either of a flow rate and a pressure of the combustion gas is controlled by a throttle means 11 related to a means such as a valve 8 which can selectively close the special port.