An expander including an expansion mechanism (50, 100, 200), which includes a first member (71, 81, 102, 112, 210) and a second member (75, 85, 116, 124, 220), which are in eccentric and relative movement, and generates power by expansion of fluid in a fluid chamber (72, 82, 230) formed between the members, wherein a main suction hole (55, 103, 201) connecting the fluid chamber (72, 82, 230) to a suction passage first in a suction stroke, and an auxiliary suction hole (56, 104, 113, 114, 203, 204, 205) connecting the fluid chamber (72, 82, 230) to a suction passage after connection of the main suction hole (55, 103, 201) are formed in the expansion mechanism (50, 100, 200), in the expansion mechanism (50, 100, 200) the fluid chamber (72, 82, 230) is defined so that at least a suction stroke and a discharge stroke are performed independently an on-off valve is provided in the suction passage connected to the auxiliary suction hole (56, 104, 113, 114, 203, 204, 205), further comprising on-off valve control means configured to control the on-off valve provided in the suction passage connected to the auxiliary suction hole (56, 104, 113, 114, 203, 204, 205) on the basis of a pressure of the fluid introduced in the expansion mechanism (50, 100, 200), characterized in that a plurality of auxiliary suction holes (56, 104, 113, 114, 203, 204, 205) are provided in the expansion mechanism (50, 100, 200), and on-off valves are provided in the suction passages connected to the auxiliary holes, when the pressure is larger than a target value, the on-off valve control means opens the on-off valves sequentially so that the fluid chamber (72, 82, 230) is connected sequentially to the suction passages through the auxiliary suction holes (56, 104, 113, 114, 203, 204, 205) and when the pressure is smaller than a target valve, the on-off value control means controls the on-off valves to sequentially close the auxiliary suction holes (56, 104, 113, 114, 203, 204, 205) in reverse order of connection from one connected to the fluid chamber (72, 82, 230) last. a main suction hole (55, 103, 201) connecting the fluid chamber (72, 82, 230) to a suction passage first in a suction stroke, and an auxiliary suction hole (56, 104, 113, 114, 203, 204, 205) connecting the fluid chamber (72, 82, 230) to a suction passage after connection of the main suction hole (55, 103, 201) are formed in the expansion mechanism (50, 100, 200), in the expansion mechanism (50, 100, 200) the fluid chamber (72, 82, 230) is defined so that at least a suction stroke and a discharge stroke are performed independently an on-off valve is provided in the suction passage connected to the auxiliary suction hole (56, 104, 113, 114, 203, 204, 205), further comprising on-off valve control means configured to control the on-off valve provided in the suction passage connected to the auxiliary suction hole (56, 104, 113, 114, 203, 204, 205) on the basis of a pressure of the fluid introduced in the expansion mechanism (50, 100, 200), characterized in that a plurality of auxiliary suction holes (56, 104, 113, 114, 203, 204, 205) are provided in the expansion mechanism (50, 100, 200), and on-off valves are provided in the suction passages connected to the auxiliary holes, when the pressure is larger than a target value, the on-off valve control means opens the on-off valves sequentially so that the fluid chamber (72, 82, 230) is connected sequentially to the suction passages through the auxiliary suction holes (56, 104, 113, 114, 203, 204, 205) and when the pressure is smaller than a target valve, the on-off value control means controls the on-off valves to sequentially close the auxiliary suction holes (56, 104, 113, 114, 203, 204, 205) in reverse order of connection from one connected to the fluid chamber (72, 82, 230) last.