To provide an engine supercharger which prevents an adverse effect of a delay in supercharging caused by an intercooler in an engine using a turbocharger loaded on construction machinery, namely smoke and deterioration of fuel consumption.

The engine supercharger is equipped with a supercharger provided to a intake path of an engine and an intercooler provided to the path, wherein the by-pass path which does not pass through the intercooler is provided to the downstream path of the supercharger. A switching valve is mounted in the inlet and outlet of the by-pass path or on the path so as to close the by-pass path when intake pressure of the engine exceeds the predetermined value.