PURPOSE: To prevent printing beyond the width of a label or its paper mount at low cost, by detecting the presence or absence of the label or both ends thereof in the direction perpendicular to its feeding direction using a scanning means provided for verifying a printed indicator.

CONSTITUTION: It is detected whether a label paper 14 is present by making an optical head 109 scan laterally and, if not, the label paper 14 is moved until it is detected. The optical head 109 is then moved to the center of the detected label paper 14 and the label paper 14 is fed to a point where the absence of a label 14b is detected and, if it is again detected from a place where the label 14b is not detected, the upper end of the label 14b in the feeding direction is recognized and its feeding is once stopped and the optical head 109 is made to scan laterally, whereby each side of the label 14b is recognized and its width is computed. The optical head 109 is thereafter brought back to its central position and the label paper 14 is fed until the label 14b is not detected, thereby recognizing the lower end of the label 14a in the feeding direction. The height H of the label 14b is then computed based on the recognized upper and lower ends of the label paper 14 and the feeding speed thereof.