Abstract: In a thermal transfer printer driven by a battery power source, to perform a print operation, the remaining capacity level of the battery power source (remaining battery capacity level) is detected by a voltage detector immediately before paper is fed out of a paper cassette and transferred for printing. The detected remaining battery capacity level is compared with a criterion level, which is a print-driving power required for one sheet of paper. If the remaining battery capacity level is lower than the criterion level, control is performed to stop the commencement of a print-driving operation. Thereby, the battery-driven printer averts printing from being terminated partway because of depletion in the battery power, and furthermore, enables printed sheets of the paper to utmost be increased.