PURPOSE: To restrain dilution gas from being taken in a cylinder by opening a switch valve after a changeover from a high speed mode to a low speed mode at the time of transition from a low suction air quantity region to a high suction air quantity region.

CONSTITUTION: A control unit 45 controls an exhaust cut valve 25 to a low speed mode in a high suction air quantity region determined on the basis of an engine rotation number and a throttle valve opening degree and to a high speed mode in a low suction air quantity region. At the time of transition from the low suction air quantity region to the high suction air quantity region, a switch valve 41 is opened after the exhaust cut valve 25 is changed from the high flow speed mode to the low flow speed mode. The area of the exhaust gas passage can be increased in the low flow speed mode so that the exhaust pressure on the upstream side of a turbine can be decreased. A secondary suction port 6 is not communicated until a difference between the exhaust pressure of an exhaust port 7 and the suction pressure of suction ports 5, 6 becomes small. Therefore, dilution gas can be restrained from being taken in a cylinder 2.