An internal combustion engine has a mechanical super-charger in a conduit connecting the engine with a source of air for compressing the air in the conduit. A bypass passage connected to the conduit on opposite sides of the super-charger compressor is selectively closed by a pressure regulating valve having a valve member movable in response to a pressure differential on opposite sides of a diaphragm member. The diaphragm member is sensitive to a pressure change in the conduit occurring in response to movement of the throttle valve, and causes the valve member to open and reduce the load on the super-charger compressor when the throttle valve is closed.