During a transient period, the opening degree of a throttle valve (throttle opening degree) is varied from a steady-period target throttle opening degree in a region A1 toward a valve closing side by a predetermined amount ΔP, and is thereafter controlled so as to become a steady-period target throttle opening degree in the region A1. The transient period is a transient period in which the operation state is shifted from a region B2 in which an air-fuel ratio in a supercharged state becomes a predetermined lean air-fuel ratio to a region A1 in which the air-fuel ratio in a non-supercharged state becomes a predetermined rich air-fuel ratio richer than the lean air-fuel ratio. In this transient period, by reducing the air amount in a cylinder, the combustion torque of an internal combustion engine is suppressed, and consequently; a torque overshoot can be suppressed.