PURPOSE: To provide mixture suited for respective running conditions while improving fuel consumption by forming a helical type intake port and supplying lean mixture in idling, mixture with theoretical air fuel ratio in low load and rich mixture in high load.

CONSTITUTION: A controlling unit 27 receiving detected values from an O2 sensor 26, idle switch 37, negative pressure sensor 112, etc. controls an atmosphere releasing valves 54, 63 for opening and closing first and second power valves 42, 43, electromagnetic change-over valve 81 for controllably opening and closing EGR controlling valve 71, electromagnetic change-over valve 94 for controlling ignition timing and electromagnetic change-over valve 107 for pivoting a rotary valve 18 for opening and closing a helical port branching path 14. And it controls the respective change-over valves to supply lean mixture in -500mmHgW -250mmHg of negative pressure, mixture with theoretical air fuel ratio in -250mmHgW-100mmHg and rich mixture in -100mmHg or less while opening the rotary valve 18.