To provide a fuel heating system of a diesel engine fuel supply device, always using only biodiesel fuel without causing a problem in startability, and making biodiesel fuel usable only with an add-on device without modification of the engine, in a diesel engine using the biodiesel fuel.

In the diesel engine fuel supply device provided with a fuel tank 1 and a fuel injection pump 30 injecting fuel to the engine, a fuel filter 10 and a heat exchanger 20 are provided between the fuel tank 1 and the fuel injection pump 30, an electrical heating means 40 is attached to the fuel filter 10 and the fuel injection pump 30, fuel is heated by circulating engine cooling water in the heat exchanger 20, and the fuel filter 10 and the fuel injection pump 30 are heated by the electrical heating means 40.