A positive displacement machine, comprising: a guide cylindrical member formed in a cylindrical shape; a reciprocating member comprising piston portions (42a, 42b) configured to be guided by an inner peripheral surface of the guide cylindrical member to be reciprocated in a direction of a center axis of the guide cylindrical member while swinging about the center axis; and a pair of first arm portions (44a, 44b) mounted to the piston portions (42a, 42b) to be perpendicular to the center axis of the guide cylindrical member and to be symmetrical with respect to the center axis; a pair of shaft members (50a, 50b) arranged to be perpendicular to the center axis of the guide cylindrical member and to be symmetrical with respect to the center axis; a pair of second arm portions (54a, 54b, 154a) mounted to the pair of shaft members (50a, 50b), such as to respectively support the pair of first arm portions (44a, 44b) at positions deviated from rotation axes of the pair of shaft members (50a, 50b); and working chambers (62a, 62b) configured to have a change in volume accompanied with a reciprocating motion of the piston portions (42a, 42b), wherein the second arm portions (54a, 54b, 154a) support the first arm portions (44a, 44b) such that a predetermined specific point (P1a) in the first arm portions (44a, 44b) is constrained in a movable manner on an axis parallel to the rotation axis of the shaft member; and the first arm portions (44a, 44b) include outer peripheral spherical portions (45a, 45b) having a center of sphere as the specific point (P1a), characterized in that either the second arm portions (54a, 54b) include an inner peripheral cylindrical portion that is arranged on the axis parallel to the rotation axis of the shaft members (50a, 50b) and are configured to hold the outer peripheral spherical portions (45a, 45b) in a slidable manner, or the second arm portions include an inner peripheral spherical portion that is configured to hold the outer peripheral spherical portion (45a, 45b) and to move on the axis parallel to the rotation axis of the shaft members (50a, 50b). a guide cylindrical member formed in a cylindrical shape; a reciprocating member comprising piston portions (42a, 42b) configured to be guided by an inner peripheral surface of the guide cylindrical member to be reciprocated in a direction of a center axis of the guide cylindrical member while swinging about the center axis; and a pair of first arm portions (44a, 44b) mounted to the piston portions (42a, 42b) to be perpendicular to the center axis of the guide cylindrical member and to be symmetrical with respect to the center axis; a pair of shaft members (50a, 50b) arranged to be perpendicular to the center axis of the guide cylindrical member and to be symmetrical with respect to the center axis; a pair of second arm portions (54a, 54b, 154a) mounted to the pair of shaft members (50a, 50b), such as to respectively support the pair of first arm portions (44a, 44b) at positions deviated from rotation axes of the pair of shaft members (50a, 50b); and working chambers (62a, 62b) configured to have a change in volume accompanied with a reciprocating motion of the piston portions (42a, 42b), wherein the second arm portions (54a, 54b, 154a) support the first arm portions (44a, 44b) such that a predetermined specific point (P1a) in the first arm portions (44a, 44b) is constrained in a movable manner on an axis parallel to the rotation axis of the shaft member; and the first arm portions (44a, 44b) include outer peripheral spherical portions (45a, 45b) having a center of sphere as the specific point (P1a), characterized in that either the second arm portions (54a, 54b) include an inner peripheral cylindrical portion that is arranged on the axis parallel to the rotation axis of the shaft members (50a, 50b) and are configured to hold the outer peripheral spherical portions (45a, 45b) in a slidable manner, or the second arm portions include an inner peripheral spherical portion that is configured to hold the outer peripheral spherical portion (45a, 45b) and to move on the axis parallel to the rotation axis of the shaft members (50a, 50b).