A medical arm assembly (1, 2, 3, 4, 5, 6, 7, 8) comprising: a remote joint in which a remote point spaced apart from a reference point is located; operating arm section connected to the remote joint, the operating arm section being configured to fix a medical tool thereto and rotate the tool about a remote rotation axis that is perpendicular to a virtual reference straight line passing through the reference point and the remote point and passes through the remote point, the operating arm section being configured to move the tool in a direction that is perpendicular to the remote rotation axis and passes through the remote point; and an arm supporting section, to which the operating arm section is connected to be supported, configured to be rotatable about the reference straight line, the reference point being located in the arm supporting section, characterized in that the medical arm assembly (1, 2, 3, 4, 5, 6, 7, 8) further comprises a positioning arm section connected to the arm supporting section to be supported, the positioning arm section being configured to support the remote joint, move the remote joint such that a relative position of the remote point with respect to the reference point is changed only in a direction of the reference straight line, and fix the relative position of the remote point with respect to the reference point, and the operating arm section includes: an ending link slidably supported on the remote joint and configured to be rotatable about the remote rotation axis and to be movable in a direction that is perpendicular to the remote rotation axis and passes through the remote point, a middle link connected to the ending link so as to be rotatable about a rotation axis parallel to the remote rotation axis; and a starting link connected to the middle link so as to be rotatable about a rotation axis parallel to the remote rotation axis and connected to the arm supporting section so as to be rotatable about a rotation axis parallel to the remote rotation axis. a remote joint in which a remote point spaced apart from a reference point is located; operating arm section connected to the remote joint, the operating arm section being configured to fix a medical tool thereto and rotate the tool about a remote rotation axis that is perpendicular to a virtual reference straight line passing through the reference point and the remote point and passes through the remote point, the operating arm section being configured to move the tool in a direction that is perpendicular to the remote rotation axis and passes through the remote point; and an arm supporting section, to which the operating arm section is connected to be supported, configured to be rotatable about the reference straight line, the reference point being located in the arm supporting section, characterized in that the medical arm assembly (1, 2, 3, 4, 5, 6, 7, 8) further comprises a positioning arm section connected to the arm supporting section to be supported, the positioning arm section being configured to support the remote joint, move the remote joint such that a relative position of the remote point with respect to the reference point is changed only in a direction of the reference straight line, and fix the relative position of the remote point with respect to the reference point, and the operating arm section includes: an ending link slidably supported on the remote joint and configured to be rotatable about the remote rotation axis and to be movable in a direction that is perpendicular to the remote rotation axis and passes through the remote point, a middle link connected to the ending link so as to be rotatable about a rotation axis parallel to the remote rotation axis; and a starting link connected to the middle link so as to be rotatable about a rotation axis parallel to the remote rotation axis and connected to the arm supporting section so as to be rotatable about a rotation axis parallel to the remote rotation axis. an ending link slidably supported on the remote joint and configured to be rotatable about the remote rotation axis and to be movable in a direction that is perpendicular to the remote rotation axis and passes through the remote point, a middle link connected to the ending link so as to be rotatable about a rotation axis parallel to the remote rotation axis; and a starting link connected to the middle link so as to be rotatable about a rotation axis parallel to the remote rotation axis and connected to the arm supporting section so as to be rotatable about a rotation axis parallel to the remote rotation axis.