PURPOSE: To enhance maximum torque by installing a variable orifice in a pressure chamber of a supercharging pressure control unit for an engine with a supercharger and by increasing the number of revolution and reducing the communication to ambient air and by increasing a supercharging pressure in a region of medium speed revolution.

CONSTITUTION: A bypass channel 6 is provided to bypass a turbine 4A of a super- charger 3, while a waste gate valve 7 is provided in the channel 6. A charging pressure in the lower stream of a compressor 4B is introduced into a pressure chamber 10A through a pipe 11, and a valve body 8 is opened as the number of revolution is increased. The pressure chamber 10A is provided with a variable orifice 13 which is communicated to ambient air. The orifice 13 further allows slidable insertion of a peedle valve 14 which reduces the area of opening by allowing a fuel pressure of an injection pump 18 to act on the pressure chamber 16A. As the number of revolution of engine increases, the super-charging pressure also rises. Up to the region of intermediate rotary speed, however, the pressure inside the pressure chamber 10A for a controller 9 leaks outside from the orifice 13, subsequently the valve body 8 is not opened, and the supercharging pressure increases.