PURPOSE: To prevent a printing head of a thermal printer from being damaged by burning because of erroneous selection of paper and printing density by making automatic adjustment of printing density available for the printing head.

CONSTITUTION: A test pattern printing part 3 executes printing of test patterns for printing density adjustment. Analysis of printing density and judgement for optimum value are executed with a printing density measurement sensor in a printing density analysis part 4, and optimum printing density data are stored in a printing density data storing part 5. A printing density control part 6 controls, based on the stored data, so that printing 7 by normal printing operation can be executed with the optimum printing density.