A rotary expander comprising: a cylinder having an inner circumferential surface that forms a cylindrical surface; a piston disposed inside the cylinder to form a working chamber between the piston and the inner circumferential surface, the piston moving along the inner circumferential surface; closing members (65, 66) for closing the working chamber with the cylinder being sandwiched therebetween; a suction passage (65a, 65b, 33c) for allowing a working fluid to flow into the working chamber; a shaft having an eccentric portion to which the piston is fitted, the shaft receiving a rotational force by expansion of the working fluid that has flowed into the working chamber; a discharge passage (67B, 61c) for allowing the expanded working fluid to be discharged from the working chamber; and a partition member for partitioning the working chamber into a suction-side working chamber and a discharge-side working chamber, the partition member being held by the cylinder, characterized in that the rotary expander is a single stage rotary expander comprising the cylinder as a single cylinder, the rotary expander further comprises an injection passage (67C, 65d), provided with an opening degree adjustable throttle valve, for introducing further the working fluid into the working chamber in an expansion process of the working fluid, and an introduction outlet of the injection passage (67C, 67d) leading to the working chamber is provided at a position on one of the closing members (65, 66), the position being located inwardly away from the inner circumferential surface of the cylinder in such a manner that the injection passage (67C, 67d) and the discharge passage (67B, 61c) are not communicated with each other and the injection passage (67C, 65d) is allowed to open only into the suction-side working chamber by opening and closing of the introduction outlet by the movement of the piston. a cylinder having an inner circumferential surface that forms a cylindrical surface; a piston disposed inside the cylinder to form a working chamber between the piston and the inner circumferential surface, the piston moving along the inner circumferential surface; closing members (65, 66) for closing the working chamber with the cylinder being sandwiched therebetween; a suction passage (65a, 65b, 33c) for allowing a working fluid to flow into the working chamber; a shaft having an eccentric portion to which the piston is fitted, the shaft receiving a rotational force by expansion of the working fluid that has flowed into the working chamber; a discharge passage (67B, 61c) for allowing the expanded working fluid to be discharged from the working chamber; and a partition member for partitioning the working chamber into a suction-side working chamber and a discharge-side working chamber, the partition member being held by the cylinder, characterized in that the rotary expander is a single stage rotary expander comprising the cylinder as a single cylinder, the rotary expander further comprises an injection passage (67C, 65d), provided with an opening degree adjustable throttle valve, for introducing further the working fluid into the working chamber in an expansion process of the working fluid, and an introduction outlet of the injection passage (67C, 67d) leading to the working chamber is provided at a position on one of the closing members (65, 66), the position being located inwardly away from the inner circumferential surface of the cylinder in such a manner that the injection passage (67C, 67d) and the discharge passage (67B, 61c) are not communicated with each other and the injection passage (67C, 65d) is allowed to open only into the suction-side working chamber by opening and closing of the introduction outlet by the movement of the piston.