A rotary expander includes: a cylinder (61); a piston (62) disposed inside the cylinder (61); closing members disposed with the cylinder (61) being sandwiched therebetween; and an injection passage for introducing further a working fluid in the expansion process of the working fluid. An introduction outlet (65c) of the injection passage leading to the working chamber (69) is provided at a position located inwardly away from the inner circumferential surface (61b) of the cylinder (61), on one of the closing members, in such a manner that the injection passage and the discharge passage are not communicated with each other.