PURPOSE: To eliminate irregularity of color density according to the printing direction of a both-direction printing head, by a method wherein a common electrode is provided on the center line of heating elements which are placed on the outside peripheral surface of a cylindrical head made of glass or the like, and heat is generated by passing an electric current through the element on either side of the center line.

CONSTITUTION: A common electrode element 5 is provided on the center line of the heating elements 21W2n, provided on the outside peripheral surface of a cylindrical head 1 made of glass or the like, and individual electrodes 611W61n, 621W62n are provided in correspondence to the heating elements 21W2n to constitute the head 1. When the center line of the head 1 is maintained to the orthogonal to a thermal printing paper 3 and a platen 4, printing in a printing direction (+) is performed by the electrode provided at the position corresponding to a head inclination angle of +θ0 (curve E), while the printing in the opposite printing direction (-) is performed by the electrode provided at the position corresponding to a heat inclination angle of -θ0 (curve F). In both cases of printing, the maximum color density Dmax is obtained. When there is a stagger of an angle Δθ to the direction (+), the printing in the direction (+) is performed in the condition corresponding to a point R, while the printing in the direction (-) is performed in the condition corresponding to a point S, and the difference in color density is extremely little.