To permit a smooth transition into a driving state in an internal combustion engine with gas changing valves and a fuel supply part switchable to partial cylinders by switching the gas changing valves into a cut-in state, and after inspecting the switch-over, to make the fuel supply part cut-in to the previously cut-out cylinders.

In a six-cylinder horizontally opposed engine 7 with cylinders 1-3 forming a first bank 8 and with cylinders 4-6 forming a second band 9, the second bank 9 is provided with gas changing valves switched only when hydraulic control pressure is formed from a hydraulic pressure supply part through a hydraulic device valve 10. During the operation of the engine 7, a control device 11 forms a switching signal to the hydraulic device valve 10 on the basis of an idling signal, a rotating speed signal, an engine temperature signal, a load signal and exhaust gas composition signal. After the gas changing valves are cut in when necessary, whether the gas changing valves are switched is inspected, and a fuel supply part to the previously cut-out cylinders is cut in after obtaining the inspected result.