A thermal printhead having an optimal resistor element shape which is capable of high speed and high quality printing is provided. The thermal printhead for progressively printing a pattern of dots on a thermosensitive paper moving in one direction, includes a substrate, and a plurality of resister elements on the substrate arranged in a substantially straight line extending in a direction perpendicular to a moving direction of the thermosensitive paper, the plurality of resistor elements each corresponding to respective one of the dots constituting the pattern to be printed onto the thermosensitive paper, each of the resistor elements having a heat transfer area that will be in contact with the thermosensitive paper upon printing the dot, wherein at least one of two sides of the heat transfer area that are substantially perpendicular to the moving direction of the thermosensitive paper has a sag in a substantially middle portion thereof to adjust a shape of the dot.