A thermal head driving IC for supplying voltage to a plurality of heating resistors each controlled by a driving MOS transistor includes a switch for making and breaking electrically between a substrate and a source of the plurality of driving MOS transistors. In a case where the plurality of heating resistors are activated, the plurality of driving MOS transistors are turned on and the switch is turned off, whereby the substrate potential is floated. As a result, the substrate potential is forward-biased against the source by a substrate current generated in a high-electric-field depletion region near the drain, and a parasitic bipolar transistor turns on, whereby both the plurality of driving MOS transistors and the parasitic bipolar transistor turn on. In a case where the plurality of heating resistors are not activated, a signal for turning off the plurality of driving NMOS transistors is given, and the switch is turned on.