A method for controlling a printing speed of a thermal head having a plurality of heater elements arranged in a line includes: dividing a set of printing data into blocks each comprising one or more printing lines aligned in a main scanning direction, in which the dividing is in a sub scanning direction, and in which a block including a first predetermined number of printing lines is regarded as a unit block; calculating the number of the heater elements to be driven for each of printing lines included in a block as the number of on-dots, and comparing the numbers of on-dots for the block to obtain a maximum number of on-dots for the block; and setting a printing speed for one block to be slower than a printing speed for another block with smaller maximum number of on-dots than maximum number of on-dots for the one block.