PURPOSE: To make it possible to automatically set printing density to proper one only by one-time printing a pattern, by printing a predetermined pattern of which the printing density was changed stepwise and detecting and storing a printing condition for making density proper to perform subsequent printing under said printing condition.

CONSTITUTION: In a thermal label printer, a label sensor detects not only the position of a label but also the printing density thereof. When printing density is adjusted, the duty of a thermal head is changed and, for example, at every feed of 30 dots with respect to 5 stages AWE, printing is performed while density is changed to each stage and developed color density is detected by the label sensor 4 and detected voltage is compared with set voltage VB capable of obtaining proper developed color density. For example, when developed color density is B and detected voltage exceeds set voltage VB, printing is performed in this duty ratio hereinafter.