A thick film thermal head includes a substrate which is provided with a groove on a surface to extend in a main scanning direction and has an electrically conductive portion which faces the groove and extends substantially over the entire length of the groove. A resistance heater strip is embedded in the groove to be in contact with the electrically conductive portion substantially over its entire length. A plurality of discrete electrodes are formed on the surface of the substrate and are in contact with the resistance heater strip at predetermined intervals in the main scanning direction. The discrete electrodes are electrically insulated from the electrically conductive portion of the substrate except through the resistance heater strip, and the electrically conductive portion is connected to a power source to be applied with an electrical potential and forms a common electrode. The discrete electrodes are connected to the power source through respective switching means to be selectively supplied with an electrical potential different from that applied to the electrically conductive portion.