PURPOSE: To control a gap between labels by emitting a light to a one surface of a mounting sheet material along a printing route, detecting transmittance of the light transmitting the material, deciding whether the light is transmitted only through a mounting sheet or between both the sheet and a printing medium, and outputting a designating signal.

CONSTITUTION: An array of light emitting diodes 32 is disposed to cross a printing route 38, and a light for transmitting a mounting sheet material is emitted to the material to be fed from a printing medium material 40. The light transmitting a mounting sheet is captured by an optical fiber 140, sent to a photodiode to convert an amount of a transmitted light into an analog electric signal. This signal is changed according to whether the light is transmitted only through the material or both the sheet and the medium. Accordingly, this analog signal is converted into a digital signal, processed by a microprocessor, and a designating signal for controlling advancing of the medium and the material passing through a printer in response to a transmitting state of the light is output.