An engine operable in a premixed combustion system, in which gaseous fuel mixed with air is caused to flow into a combustion chamber, and a diffusion combustion system that injects liquid fuel into the combustion chamber for combustion, the engine comprising: an engine body configured to generate a driving force using supplied fuel; a gaseous fuel tank for storing gaseous fuel in a liquefied state; a gaseous fuel supply path connected to the gaseous fuel tank and configured to supply fuel gas to the engine body; a liquid fuel tank for storing liquid fuel; a liquid fuel supply path connected to the liquid fuel tank and configured to supply liquid fuel to the engine body; a main fuel injection valve configured to supply liquid fuel to the combustion chamber during combustion in the diffusion combustion system; a pilot fuel injection valve configured to supply liquid fuel as pilot fuel to the combustion chamber in order to ignite gaseous fuel during combustion in the premixed combustion system; wherein the liquid fuel supply path comprises: a main fuel supply path configured to supply liquid fuel stored in the liquid fuel tank to the main fuel injection valve; a pilot fuel supply path configured to supply liquid fuel stored in the liquid fuel tank as pilot fuel to the pilot fuel injection valve; a pilot fuel filter disposed in an intermediate portion of the pilot fuel supply path; a pilot fuel high-pressure pump disposed in an intermediate portion of the pilot fuel supply path and configured to send, to the pilot fuel injection valve, pilot fuel that has passed through the pilot fuel filter; a pilot fuel tank disposed in the intermediate portion of the pilot fuel supply path and configured to store pilot fuel sent from the pilot fuel high-pressure pump and not injected by the pilot fuel injection valve; and a pilot fuel supply pump disposed in an intermediate portion of the pilot fuel supply path and configured to send pilot fuel stored in the pilot fuel tank to the pilot fuel filter, characterized in that the engine further comprises a fuel purification path, which is a path for circulation through a purification filter for purifying pilot fuel and the pilot fuel tank. an engine body configured to generate a driving force using supplied fuel; a gaseous fuel tank for storing gaseous fuel in a liquefied state; a gaseous fuel supply path connected to the gaseous fuel tank and configured to supply fuel gas to the engine body; a liquid fuel tank for storing liquid fuel; a liquid fuel supply path connected to the liquid fuel tank and configured to supply liquid fuel to the engine body; a main fuel injection valve configured to supply liquid fuel to the combustion chamber during combustion in the diffusion combustion system; a pilot fuel injection valve configured to supply liquid fuel as pilot fuel to the combustion chamber in order to ignite gaseous fuel during combustion in the premixed combustion system; wherein the liquid fuel supply path comprises: a main fuel supply path configured to supply liquid fuel stored in the liquid fuel tank to the main fuel injection valve; a pilot fuel supply path configured to supply liquid fuel stored in the liquid fuel tank as pilot fuel to the pilot fuel injection valve; a pilot fuel filter disposed in an intermediate portion of the pilot fuel supply path; a pilot fuel high-pressure pump disposed in an intermediate portion of the pilot fuel supply path and configured to send, to the pilot fuel injection valve, pilot fuel that has passed through the pilot fuel filter; a pilot fuel tank disposed in the intermediate portion of the pilot fuel supply path and configured to store pilot fuel sent from the pilot fuel high-pressure pump and not injected by the pilot fuel injection valve; and a pilot fuel supply pump disposed in an intermediate portion of the pilot fuel supply path and configured to send pilot fuel stored in the pilot fuel tank to the pilot fuel filter, characterized in that the engine further comprises a main fuel supply path configured to supply liquid fuel stored in the liquid fuel tank to the main fuel injection valve; a pilot fuel supply path configured to supply liquid fuel stored in the liquid fuel tank as pilot fuel to the pilot fuel injection valve; a pilot fuel filter disposed in an intermediate portion of the pilot fuel supply path; a pilot fuel high-pressure pump disposed in an intermediate portion of the pilot fuel supply path and configured to send, to the pilot fuel injection valve, pilot fuel that has passed through the pilot fuel filter; a pilot fuel tank disposed in the intermediate portion of the pilot fuel supply path and configured to store pilot fuel sent from the pilot fuel high-pressure pump and not injected by the pilot fuel injection valve; and a pilot fuel supply pump disposed in an intermediate portion of the pilot fuel supply path and configured to send pilot fuel stored in the pilot fuel tank to the pilot fuel filter, characterized in that the engine further comprises a fuel purification path, which is a path for circulation through a purification filter for purifying pilot fuel and the pilot fuel tank.