To simply and surely carry out calibration of a printer by visual observation without using a densitometer.

A Y reference pattern 13, an M reference pattern 14, and a C reference pattern 15 to be printing density criteria are printed in the form of gradations at a leading end part of a long color thermal recording sheet. A color thermal printer forms a sample print 21 by solid printing a Y calibration pattern 18, an M calibration pattern 19 and a C calibration pattern 20 of predetermined optical density values adjacently above each of the reference patterns 13-15. The user observes the sample print 21, selects a part approximate to a printing density of each of the calibration patterns 18-20 among each of the reference patterns 13-15, and specifies a density number printed below the selected part. When the density number is inputted to the color thermal printer, density correction for every color is carried out automatically.