Abstract: A printer devised so as to function both as a thermal printer and as an ink-jet printer. The printer having a thermal printing head cartridge, an ink-jet printing head cartridge, a cartridge holder, an electronic circuit and a sensor for identifying a printing head cartridge in use. If one of the above two kinds of printing head cartridges is optionally selected and mounted on the cartridge holder, the sensor outputs a signal of identifying which one of the printing head cartridges is mounted. Receiving the signal, the electronic circuit supplies to a printing head cartridge mounted on the cartridge holder pulse currents suitable for the specific resistors installed in the printing head cartridge for making the same function as a printing head.