Startability of an internal combustion engine using a mixed fuel of gasoline and alcohol is improved. An intake passage of the engine has a main flow channel provided with a throttle valve and a bypass flow channel for bypassing the throttle valve. A bypass quantity adjusting mechanism adjusts the bypass quantity in the bypass flow channel. A control device detects or estimates the mixing ratio of alcohol of the fuel and controls the bypass quantity adjusting mechanism so that a bypass quantity before a first explosion is smaller than a bypass quantity after the first explosion when the mixing ratio of alcohol is equal to or more than a predetermined value.