Abstract: An expandable and contractable robot arm is vertically provided on a machine base through a first universal joint, with X and Y horizontal axes being defined passing through the center of rotation of the joint crossing each other at right angles and a vertical Z axes being defined as a longitudinal axis of the arm passing through the center of rotation of the joint. A pair of driving sources are provided on the machine base and connected to the arm for causing the arm to incline in the X axis direction along an X-Z plane define by the X and Z axes and in the Y axis direction along a Y-Z plane defined by the Y and Z axes. A tool attaching plate is mounted on the upper end of the arm through a second universal joint having its center of rotation positioned on the Z axis. A parallel ruler mechanism is provided between the attaching plate and the machine base so as to keep the posture of the attaching plate constant.