To easily manage and treat a working fluid and a lubricating oil to be circulated while maintaining efficiency in a scroll type expander and a Rankine cycle power generation system in which the expander is used.

This scroll type expander 2 comprises a fixed scroll 51 in which spiral scroll laps 62, 72 are formed in the end faces 61a, 71a of scroll base plates 61, 71 and a swing scroll 52. The swing scroll 52 is revolved around the axis of a drive shaft by the expansion of a gas phase working medium fed into an expansion chamber 40 formed by the scroll laps 62, 72 and the end faces 61a, 71a. First and second liquid phase working medium feed grooves 120, 121 are formed in the axial end faces 62a, 72a of the scroll laps 62, 72. First and second working medium inlets 106, 107 communicating with the first and second liquid phase working medium feed grooves 120, 121 are formed in the fixed scroll 51 and communicated with a liquid phase working medium feed part.