It is an object of the present invention to reduce the constraint that the density ratio is constant as small as possible, and to obtain high power recovering effect in a wide operation range by using an expander which is operated in accordance with a flowing direction of refrigerant. An expander used in a refrigeration cycle uses carbon dioxide as refrigerant and has a compressor, an outdoor heat exchanger and an indoor heat exchanger. The expander comprises a cylindrical cylinder, a rotor which rotates in the cylinder, a vane which divides an expansion space formed between an inner peripheral surface of the cylinder and an outer peripheral surface of the rotor into a plurality of spaces, and a vane groove provided in the rotor for accommodating the vane therein. The vane groove is provided with a back pressure chamber which pushes the vane against the inner peripheral surface of the cylinder, and the refrigerant in the supercritical state is introduced into the back pressure chamber.