A fluid machine in which: an expansion mechanism for producing power by the expansion of fluid, a compression mechanism for compressing fluid, and a rotating shaft for transmitting power produced in the expansion mechanism to the compression mechanism are housed in a container-shaped casing; and fluid discharged from the compression mechanism is fed to the outside of the casing by way of an internal space defined in the casing;wherein: lubricating oil is stored on the side of the compression mechanism in the inside of the casing; and the fluid machine comprises: an oil supply passageway which is formed in the rotating shaft and which supplies lubricating oil stored in the inside of the casing to the expansion mechanism and has a terminating end from which surplus lubricating oil is discharged; and an oil return passageway for guiding the surplus lubricating oil towards the compression mechanism from the terminating end of the oil supply passageway, an electric motor, coupled to the rotating shaft to drive the compression mechanism, is arranged between the compression mechanism and the expansion mechanism in the casing, a discharge pipe, through which fluid discharged from the compression mechanism is led out to the outside of the casing, is arranged between the electric motor and the expansion mechanism in the casing, the fluid machine further includes an oil return pipe having a starting end connected to the expansion mechanism, and forming part of the oil return passageway, characterized in that the oil return pipe has a terminating end entering a clearance defined between a core cut part formed in an outer periphery of a stator of the electric motor and the casing, and the terminating end of the oil return pipe forms an terminating end of the oil return passageway. an expansion mechanism for producing power by the expansion of fluid, a compression mechanism for compressing fluid, and a rotating shaft for transmitting power produced in the expansion mechanism to the compression mechanism are housed in a container-shaped casing; and fluid discharged from the compression mechanism is fed to the outside of the casing by way of an internal space defined in the casing;wherein: lubricating oil is stored on the side of the compression mechanism in the inside of the casing; and the fluid machine comprises: an oil supply passageway which is formed in the rotating shaft and which supplies lubricating oil stored in the inside of the casing to the expansion mechanism and has a terminating end from which surplus lubricating oil is discharged; and an oil return passageway for guiding the surplus lubricating oil towards the compression mechanism from the terminating end of the oil supply passageway, an electric motor, coupled to the rotating shaft to drive the compression mechanism, is arranged between the compression mechanism and the expansion mechanism in the casing, a discharge pipe, through which fluid discharged from the compression mechanism is led out to the outside of the casing, is arranged between the electric motor and the expansion mechanism in the casing, the fluid machine further includes an oil return pipe having a starting end connected to the expansion mechanism, and forming part of the oil return passageway, characterized in that the oil return pipe has a terminating end entering a clearance defined between a core cut part formed in an outer periphery of a stator of the electric motor and the casing, and the terminating end of the oil return pipe forms an terminating end of the oil return passageway. lubricating oil is stored on the side of the compression mechanism in the inside of the casing; and the fluid machine comprises: an oil supply passageway which is formed in the rotating shaft and which supplies lubricating oil stored in the inside of the casing to the expansion mechanism and has a terminating end from which surplus lubricating oil is discharged; and an oil return passageway for guiding the surplus lubricating oil towards the compression mechanism from the terminating end of the oil supply passageway, an oil supply passageway which is formed in the rotating shaft and which supplies lubricating oil stored in the inside of the casing to the expansion mechanism and has a terminating end from which surplus lubricating oil is discharged; and an oil return passageway for guiding the surplus lubricating oil towards the compression mechanism from the terminating end of the oil supply passageway, an electric motor, coupled to the rotating shaft to drive the compression mechanism, is arranged between the compression mechanism and the expansion mechanism in the casing, a discharge pipe, through which fluid discharged from the compression mechanism is led out to the outside of the casing, is arranged between the electric motor and the expansion mechanism in the casing, the fluid machine further includes an oil return pipe having a starting end connected to the expansion mechanism, and forming part of the oil return passageway, characterized in that the oil return pipe has a terminating end entering a clearance defined between a core cut part formed in an outer periphery of a stator of the electric motor and the casing, and the terminating end of the oil return pipe forms an terminating end of the oil return passageway.