The thermal printer of the present invention is a thermal printer for printing on a print medium by thermally transferring an ink ribbon by means of a thermal head, wherein the thermal printer comprises a density controller for keeping the print density of the thermal head low in low-temperature control for raising the print density at low temperatures. The density controller comprises a density calculator for calculating a density evaluation value by finding an average gradation value of print data for each of a plurality of dots included in the predetermined region, a comparator for comparing the calculated density evaluation value with a predetermined value, and an adjuster for adjusting, on the basis of this comparison result, the print density in driving and printing with the thermal head to a low value for print data of high gradation exceeding a predetermined gradation value in print data on a printed line when the density evaluation value exceeds a predetermined value. The printing of each dot by the thermal head is controlled on the basis of the adjusted print density. Even if low temperature control is carried out by the thermal head when the ambient temperature is low, the occurrence of ink ribbon wrinkling due to the effect of print density is suppressed.