Sonic wave speed at a part is calculated considering the environment surrounding the part, sonic wave speed around the part is measured, and the accuracy of the position detection is improved. An ultrasonic wave transmitting device is arranged at a robot hand tip, hexahedrons which can contain the tip are set, and the ultrasonic wave transmitting-receiving devices are arranged at the top points thereof. Upon detecting the tip position, a top point that is close to the tip is selected as a specific point, and sonic wave speeds from other top points to this specific point are calculated. Based on these sonic wave speeds, sonic wave speed along a route from the tip to the specific point is calculated. Based on this sonic wave speed value and spread time of ultrasonic waves between the tip and the specific point, distance from the specific point to the tip is calculated.