To detect intake pressure on the upstream side of a supercharger without providing a new part on an engine with the mechanical supercharger.

Intake pressure on the upstream side of the supercharger 17 is computed in accordance with intake pressure on the downstream side of the supercharger 17 and engine speed in a region (region where an air by-pass valve 22 is in a totally open state) where the intake pressure on the downstream of the supercharger 17 is lower than judged pressure when a solenoid three-way valve 29 of the air by-pass valve 22 is off. Additionally, the intake pressure on the upstream side of the supercharger 17 is computed in accordance with differential pressure of the intake pressure on the downstream of the supercharger 17 and the judged pressure, atmospheric pressure and the engine speed in a region where the intake pressure on the downstream side of the supercharger 17 is higher than the judged pressure (region in the totally closed state from starting to close the air by-pass valve 22). In the meantime, the intake pressure on the upstream side of the supercharger 17 is computed in accordance with the intake pressure on the downstream side of the supercharger 17 and the engine speed at the time when the solenoid three-way valve 29 is on.