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