An internal combustion engine having cylinders capable of selectively resting during engine operation is capable of eliminating step increases in engine output when the number of operative cylinders is changed. The internal combustion engine includes a plurality of cylinders divided into two or more groups, at least some of the cylinders being restable. A throttle valve of each of the cylinders is independently operable on the basis of the group to which the cylinder belongs, and the number of resting cylinders is controlled according to the throttle grip opening. The throttle valve opening is different between the groups, except for the full opening condition and the full closure condition of the throttle valve, and an ECU is provided for opening the throttle valve of the next cylinder group before the throttle valve opening of the former cylinder group reaches the full opening condition.