A first and a second superchargers are arranged in parallel with each other on an intake air passage and an exhaust passage. At least one of the first and the second superchargers includes a variable supercharging mechanism. The control unit determines a target air amount ratio of the first and the second superchargers, and controls the variable supercharging mechanism such that an actual air amount ratio of the first and the second superchargers becomes equal to the target air amount ratio. The air amount ratio which does not cause the surge in the first and the second superchargers is set as the target air amount ratio, and the variable supercharging mechanism is controlled by using it as a target.