An evaporator includes a double pipe provided between an outer cylindrical member and an inner cylindrical member. The double pipe includes an inner pipe as a passage of water and an outer pipe as a passage of raw fuel. The inner pipe has a plurality of small holes connected to the outer pipe for maintaining the pressurized state of the water in the inner pipe. When the water gushes out of the small holes of the inner pipe in the outer pipe, phase transition of the water from the liquid phase to the gas phase occurs, and the water vapor is mixed with the raw fuel in the gas phase in the outer pipe.