A fuel supply device of an engine includes a primary pump suctioning fuel to discharge the fuel to a first pipeline, a secondary pump connected in series to the primary pump via the first pipeline, and suctioning the fuel from the primary pump to supply the fuel through a second pipeline to an engine side, a tank storing the fuel and accommodating the secondary pump, an outflow part causing part of the fuel flowing from the first pipeline through the secondary pump to the second pipeline in the tank to flow outward, a third pipeline returning, to a suction side of the primary pump, gas-liquid mixed fuel generated by vaporizing the fuel flowing out from the outflow part, and a preloading part in one of portions of a circulation path formed between the primary pump and the tank by the first pipeline and the third pipeline and preloading the fuel.