PURPOSE: To detect the amount of slippage of printing position automatically and rapidly by comparing the amount of rotation of a motor when a carrier of a printer is moved from a reference position by a predetermined distance with the amount of rotation when the carrier is returned therefrom to the reference position.

CONSTITUTION: A carrier 1 of a printer is driven by a stepping motor 2 through a belt 4 so as to be moved right and left along a guide shaft 8, wherein, when the carrier 1 is at a reference position P on the left side, the light receiving window of a photosensor 3 is intercepted by a shutter 5. Thereafter, a drive circuit 6 is controlled by a control circuit 7 to move the carrier 1 to the right side, whereby the shutter 5 opens the light receiving window so that the motor 2 is rotated through steps X of certain measured value after the time at which the sensor 3 was turn ON. And the carrier 1 is moved to the left side, whereby the measured value is taken to be steps Y. The measured values, i.e., steps X, Y, are compared and the difference therebetween is taken to be an amount of slippage (d), which can be automatically detected.