PURPOSE: To enable a thermal printer to be driven by an electric current supplied from a manganese dry cell, by a method wherein the period of time for which an electric current is passed to a heating element is determined on the basis of an output from a voltage-frequency converter.

CONSTITUTION: When charging of a capacitor 46 is started by supplying a power source 41 to the voltage-frequency converter and the voltage across the capacitor 46 reaches the base-emitter voltage of a transistor (hereinafter abbreviated to Tr) 47, the Tr 47 is turned ON, which condition is transmitted to a Tr 50 so that the Tr 50 is turned OFF. This condition is transmitted to a Tr 53 to turn the Tr 53 ON, and the voltage across the capacitor 46 is discharged. As a result, the Tr 47 is turned OFF, the Tr 50 is turned ON, the Tr 53 is turned OFF, and charging of the capacitor 46 is started. The ON-OFF operations result in ocillations with an oscillating period T approximately equal to the charging period of the capacitor 46.