To always control the viscosity of a fuel to be in a proper range by a simple structure.

A flow passage selecting valve 9 opening/closing based on a degree of a pressure loss in a fuel cooler 3 is provided in a bypass pipe line 10 provided between upstream side piping 6a and downstream side piping 6b. When the pressure in a downstream side is higher than a lower limit value in regard to the pressure in an upstream side, the flow passage selecting valve 9 is opened to directly communicate the upstream side piping 6a and the downstream side piping 6b through the bypass pipe line 10, and the fuel is supplied to an engine 1 without passing through the fuel cooler 3. When the differential pressure between the upstream side and the downstream side is lower than the lower limit value set by a spring 9a, the flow passage selecting valve 9 is closed, and the fuel is supplied to the engine 1 after being cooled by the fuel cooler 3.