A rotary machine support device (20, 40) for supporting a rotary machine which includes a bearing housing and a rotary shaft, the bearing housing including a flange (11, 62) formed outward, the rotary shaft being rotatably held inside the bearing housing, the rotary machine support device (20, 40) comprising: a support portion which supports the rotary machine, wherein the support portion includes: a mounting flange (25, 61) capable of contacting the flange (11, 62) of the rotary machine; and a plurality of arms (24, 41), each arm (24, 41) including a holding portion (32,45) which is configured to hold the mounting flange (25, 61) and the flange (11, 62) together while pressing the mounting flange (25, 61) and the flange (11, 62) inward from outside in a radial direction of the mounting flange (25, 61) in a state where the mounting flange (25, 61) contacts the flange (11, 62), the holding portion (32, 45) includes a pair of claw pieces (34a, 60) which face each other and which are capable of holding the mounting flange (25, 61) and the flange (11, 62) together, and at least one of the mounting flange (25, 61) and a groove portion which is formed between the pair of claw pieces (34a, 60) is formed in a tapered shape, and the tapered shape is formed so that a width of the tapered shape in an opposing direction of the flange (11, 62) and the mounting flange (25, 61) gradually decreases outward from inside in the radial direction of the mounting flange (25, 61), characterized in that: the holding portion (32, 45) is provided on an end of the arm (24, 41) so that the position of the holding portion (32, 45) is capable of changing in each of a direction around an axis of the arm (24, 41), a direction orthogonal to the opposing direction, and the opposing direction, relative to the flange (11, 62) and the mounting flange (25, 61). a support portion which supports the rotary machine, wherein the support portion includes: a mounting flange (25, 61) capable of contacting the flange (11, 62) of the rotary machine; and a plurality of arms (24, 41), each arm (24, 41) including a holding portion (32,45) which is configured to hold the mounting flange (25, 61) and the flange (11, 62) together while pressing the mounting flange (25, 61) and the flange (11, 62) inward from outside in a radial direction of the mounting flange (25, 61) in a state where the mounting flange (25, 61) contacts the flange (11, 62), a mounting flange (25, 61) capable of contacting the flange (11, 62) of the rotary machine; and a plurality of arms (24, 41), each arm (24, 41) including a holding portion (32,45) which is configured to hold the mounting flange (25, 61) and the flange (11, 62) together while pressing the mounting flange (25, 61) and the flange (11, 62) inward from outside in a radial direction of the mounting flange (25, 61) in a state where the mounting flange (25, 61) contacts the flange (11, 62), the holding portion (32, 45) includes a pair of claw pieces (34a, 60) which face each other and which are capable of holding the mounting flange (25, 61) and the flange (11, 62) together, and at least one of the mounting flange (25, 61) and a groove portion which is formed between the pair of claw pieces (34a, 60) is formed in a tapered shape, and the tapered shape is formed so that a width of the tapered shape in an opposing direction of the flange (11, 62) and the mounting flange (25, 61) gradually decreases outward from inside in the radial direction of the mounting flange (25, 61), characterized in that: the holding portion (32, 45) is provided on an end of the arm (24, 41) so that the position of the holding portion (32, 45) is capable of changing in each of a direction around an axis of the arm (24, 41), a direction orthogonal to the opposing direction, and the opposing direction, relative to the flange (11, 62) and the mounting flange (25, 61).