Labels can be detected on various types of label paper. Two photodetectors are disposed side by side in the transportation direction of label paper either above or below the transportation path of the printer, and a light-emitting device is disposed at a position opposite the photodetectors with the transportation path therebetween. Alternatively, the light-emitting device is disposed at a position between the photodetectors. Each time the label paper is advanced a unit amount, the light that passes through or the light that is reflected by the parts of the label paper at the detection positions on the transportation path is detected, and whether the amplified difference of these two detector outputs is greater than or equal to a specific threshold value set according to the difference in the transmittance of the liner and the label is determined. Passage of an edge part of a label or black mark between the detection positions is detected based on the result of this decision.