To provide a method for trimming the resistance of a thermal head such that a good and clear image can be formed by preventing generation of uneven density effectively.

Resistance of heating resistors arranged on the substrate of a thermal head is trimmed according to following steps 1-4. In the step 1, Joule's heat is generated from the heating resistors of the thermal head while carrying a recording medium onto the heating resistors thus forming an image on the recording medium. In the step 2, density of a print formed in Step 1 is measured. In the step 3, the print density obtained in Step 2 is compared with a target density to determine a correction width of resistance corresponding to the difference of density. In the step 4, resistance of each heating resistor is trimmed based the correction width of resistance obtained in Step 3.