*6 Department of Mechanical Engineering, Faculty of Engineering, Doshisha University, 1-3 Tataramiyakodani, Kyotanabe-shi, Kyoto, 610-0321 Japan In this study, we aim to construct the heat recovery generation system using steam Rankin cycle for cogeneration by low capacity prime mover of about 500 kW or less. As power recovery device, we focused attention on the scroll expander with high-efficiency potential. However, performance characteristics of the scroll expander for steam are not apparent. The purpose of this paper is to clarify the performance characteristics of the scroll steam expander. As the result of comparing performance characteristics of steam with that of compressed air, at the condition of constant suction pressure (300 kPa) and rotation speed of in the range from 500 rpm to 3 000 rpm, the net efficiency of compressed air is in the range of 50% to 20%, on the other hand, that of steam is in the range of 10% to 40%. Moreover, in case of steam, it was shown that the influence of the leakage loss and the heat loss are the largest.