The internal combustion engine (1) has a throttle (23) for controlling an intake air amount and performs start-up through cranking. The starter switch (36) detects cranking initiation, and the crank angle sensor (33, 34) detects the number of revolutions of the engine. The controller (31) drives the throttle (23) in a closed position along with the cranking initiation. The controller (31) can obtain both the intake negative pressure for promoting vaporization of fuel and the intake air amount necessary to maintain the idle rotation speed by counting the number of strokes or the number of revolutions of the internal combustion engine (1) from the cranking initiation and opening the throttle (23) from the closed position as the count number reaches a predetermined number.