A robot system which is inexpensive and can easily measure a position of a target point. The system stores feature quantities of an image of a target mark included in an image acquired as reference data when the target mark is placed at a known first target mark position at the robot coordinate system and stores the first target mark position with respect to an arm tip as the position of the tool center point. The system compares the feature quantities obtained from the image acquired when the target mark is arranged at a second target mark position and the feature quantities of the reference data to make the arm tip move and calculates the second target mark position at the robot coordinate system based on a second robot position corresponding to the position of the arm tip after movement and the position of the tool center point.