To precisely detect the position of a mechanically moving portion which is driven by a motor, restraining the effect of backlash. A position detector (3) produces a position detection pulse every time after a mechanically moving portion (2) driven by a motor (1) is moved by a predetermined amount. A counting means (4) counts the position detection pulses sent from the position detector (3), and a reading means (5) reads a value counted by the counting means (4) every time after a predetermined period of time. A cancelling means (6) cancels the content of the counting means (4) by a counted amount read by the reading means (5). A backlash amount storage means (7) stores a backlash amount of the drive system, and a moving amount calculating means (8) calculates a practical amount of movement of the mechanically moving portion (2) every time after said predetermined period of time based upon the content stored in the backlash amount storage means (7) and the value read by the reading means (5). An integrating means (9) integrates the results calculated by the moving amount calculating means (8).