A rotating fluid machine has first energy converting means including cylinders and pistons, and second energy converting means including vanes emerging from and submerging into a rotor in the radial direction. A gaseous phase working medium having leaked out of the higher pressure first energy converting means to a reservoir is effectively utilized instead of being discarded wastefully by feeding the leaked gaseous phase working medium to the lower pressure second energy converting means via a one-way valve and a junction chamber. Moreover, when the pressure in the reservoir is lower than that in the junction chamber, the one-way valve obstructs the gaseous phase working medium from flowing back, thereby preventing the efficiency of the second energy converting means from deteriorating.