Abstract: Resistance of a thermal head is stored as a digital signal in a head resistance memory. Output according to data in the head resistance memory is calculated by a head voltage calculation circuit for storing a correlation between resistance of the thermal head at a fixed normal temperature and supply voltage. The output from the head voltage calculation circuit is converted by a D/A converter to a reference voltage, which is then fed to a constant-voltage power source for applying the supply voltage to the thermal head, so that an optimum supply voltage to the thermal head may be automatically set, and temperature of the thermal head generating heat may be maintained constant.

Abstract: A label printer includes a printing head having a plurality of heating elements arranged in a direction perpendicular to the feed direction of a label, and a data generator for supplying a plurality of article data, including a bar code and a price, to the printing head in the ordinary printing mode. In the test mode, the data generator supplies to the printing head test pattern data indicative of a plurality of test patterns which are printed in substantially the same positions as the printing positions of those plurality of article data, each of the test patterns having an area of substantially the same size as the largest one of the areas occupied by the article data to be printed in the same printing position.