A rotary type expander is provided with two rotary mechanism parts which differ from each other in displacement volume. The outflow side of the first rotary mechanism part of small displacement volume is fluidly connected to the inflow side of the second rotary mechanism part of large displacement volume. The processes by which the volume of a first low-pressure chamber in the first rotary mechanism part decreases and the volume of a second high-pressure chamber in the second rotary mechanism part increases are respectively in sync. Refrigerant at high pressure is first introduced into a first high-pressure chamber of the first rotary mechanism part. Thereafter, this high-pressure refrigerant passes through a communicating passage and then flows by way of the first low-pressure chamber into the second high-pressure chamber while expanding. The after-expansion refrigerant flows out to an outflow port from a second low-pressure chamber of the second rotary mechanism part.