PURPOSE: To obtain printing with high image quality by keeping the voltage of an electric source to be const. to the change in the number of heat generating pixel being simultaneously energized and making thereby generated energy of each heat-generating pixel to be const.

CONSTITUTION: In a line-type thermal printer wherein a thermal head 20 wherein a number of heat generating pixels divided into a plurality of blocks are arranged in a line is provided and at every block, printing is performed while the heat generating pixels are electrically energized, a means 33 for counting a number of pixels, a means for storing the number of energized pixels, a means 12 for storing data on energizing time, means 11 and 12 for selecting energizing time and means 11 and 12 for controlling to switch energizing time are provided and in accordance with increase and decrease in the number of simultaneously energized heat generating pixels of each block, extension and shortening of the energizing time of the block is automatically adjusted.