PURPOSE: To prevent an auxiliary fit board of a hydraulic engine from being subjected to load and achieve reduction in size and weight by securing an outer casing of a combustor which is fixed to an outer race of a bearing to the auxiliary engine fit board and by firmly fitting the end of the auxiliary engine to the inner race.

CONSTITUTION: An outer casing 6 of a combustor 01 which is fixed to an outer race 10-1 of a bearing 10 is secured to an auxiliary engine fit board 09 of a hydraulic engine and the end of an auxiliary engine 09 is fixed to an inner race 10-2 of the bearing 10. The engine 08 is rotatably fit to the auxiliary engine fit board 09 via the bearing 10. Reaction, by combustion gas pressure is transmitted from the inner casing 05 of the combustor to the external cylinder 6 thereof and finally reaches the bearing from a fitting bolt 11. Another reaction by cooling water pressure is similarly transmitted from the end of the engine 08 up to the bearing 10, the same terminal. Even in the absence of the auxiliary engine fit board 09, loads by the combustion gas pressure and the cooling water pressure are all applied to the bearing 10.