To realize high-peed printing according to an optimum maximum simultaneous drive dot number under the present state for individual products without being affected by an unevenness on products and a power source state by determining the maximum simultaneous drive dot number of a line thermal head during printing.

A CPU 1 calls up an 'initial value' of the number of dots to be driven simultaneously, prints dots of an amount corresponding to the number, and judges whether or not a printer supply voltage detected by a source voltage detect part 9 during printing reduces to a printer security voltage. Unless the printer supply voltage reduces to the printer security voltage, the simultaneous drive dot number is increased and updated, and printing dots of an amount based on the updated number are repeated. When the printer supply voltage reduces to the printer security voltage, the simultaneous drive dot number at the time is set as a maximum simultaneous drive dot number, and dots of an amount based on the set maximum number are printed.