A gradation control device for a thermal ink-transfer type printing apparatus controls an applying time of a current to each of heating elements of a thermal printing head depending on a voltage drop caused to a supply voltage to the thermal printing head, so that it is possible to constantly print the same gradation level with the same printing dot size regardless of the printing data. The voltage drop is detected in a vicinity of an end of a current applying duration in which the current is supplied to the heating elements of the thermal printing head.