A rotary centrifugal compressor machine, comprising: a casing ; a rotor that includes a rotary shaft rotatably supported inside the casing , and a plurality of stages of impellers fixed to an outer periphery of the rotary shaft ; diaphragms respectively surrounding the impellers ; and gas flow paths through which process gas to be compressed flows, the gas flow paths being provided respectively corresponding to the impellers , wherein the gas flow paths include diffuser flow paths into which the process gas flowing out from the impellers to outside in a radial direction flows, curved flow paths that respectively communicate with the diffuser flow paths and change a flowing direction of the process gas from a direction toward the outside in the radial direction to a direction toward inside in the radial direction, and return flow paths that respectively communicate with the curved flow paths and cause the process gas flowing through the curved flow paths , to flow into the impellers , and the curved flow path configuring at least one of the gas flow paths is provided between the diaphragm and a flow path forming body that is provided between the diaphragm and the casing ; the rotary centrifugal compressor machine being characterised in that the casing includes an annular accommodating groove recessed outward in the radial direction, corresponding to a region provided with the flow path forming body, and the flow path forming body includes an annular shape and is mated with the accommodating groove such that at least a part of the curved flow path is formed inside the accommodating groove of the casing . a casing ; a rotor that includes a rotary shaft rotatably supported inside the casing , and a plurality of stages of impellers fixed to an outer periphery of the rotary shaft ; diaphragms respectively surrounding the impellers ; and gas flow paths through which process gas to be compressed flows, the gas flow paths being provided respectively corresponding to the impellers , wherein the gas flow paths include diffuser flow paths into which the process gas flowing out from the impellers to outside in a radial direction flows, curved flow paths that respectively communicate with the diffuser flow paths and change a flowing direction of the process gas from a direction toward the outside in the radial direction to a direction toward inside in the radial direction, and return flow paths that respectively communicate with the curved flow paths and cause the process gas flowing through the curved flow paths , to flow into the impellers , and the curved flow path configuring at least one of the gas flow paths is provided between the diaphragm and a flow path forming body that is provided between the diaphragm and the casing ; the rotary centrifugal compressor machine being characterised in that the casing includes an annular accommodating groove recessed outward in the radial direction, corresponding to a region provided with the flow path forming body, and the flow path forming body includes an annular shape and is mated with the accommodating groove such that at least a part of the curved flow path is formed inside the accommodating groove of the casing . <EOS>