The image of a tool center point (31) caught by a camera (light-receiving device) 4 from two initial positions is moved to a predetermined point, by a predetermined point moving process, at the center of a light-receiving surface thereby to acquire robot positions (Qf1, Qf2), based on which the direction of the view line (40) is determined. Next, the robot is moved to the position where the position (Qf1) is rotated by 180 degrees around the Z axis of a coordinate system (.SIGMA.v1) thereby to execute the predetermined point moving process. After rotational movement, a robot position (Qf3) is acquired. The midpoint between the position (Qf1) and the position (Qf3) is determined as the origin of a coordinate system (.SIGMA.v2). Using the position and the posture of the view line (40), the position of the tool center point (31) is determined. Thus, the position of the tool center point with respect to the tool mounting surface can be determined using a fixed light-receiving device. By additionally measuring two points at known relative positions from the tool center point, the tool posture as well as the position of the tool center point can be determined.