To uniformly maintains a recoding position by automatically adjusting and deciding a threshold value permitting the detection of the correct position of a sheet without getting influenced by a difference in the transmissivity and reflectance of the sheet caused by a difference in the kind of the sheet in a label printer.

The recording device reads the output value of an optical sensor into a CPU and outputs a threshold voltage in relation to the value by using a D/A converter. An automatic adjustment method is to intentionally move the sheet back and forth with respect to a sheet sensing sensor by a "replace button" or the like during the replacement of the sheet before the start of printing and automatically adjust the threshold value when the sheet setting is changed during printing.