Abstract: The present invention provides a method of making an auxiliary electrode layer for a common electrode pattern in a thermal printhead. The method of the present invention includes the steps of: preparing a master substrate (1') which has an obverse surface provided with a common electrode pattern (4) and corresponds to a plurality of head substrates; forming at least one slit (9) in the master substrate (1') where the slit extends along the common electrode pattern (4); and forming an auxiliary electrode layer (6) on a reverse surface of the master substrate (1') so that the auxiliary electrode layer (6) extends via the slit (9) for electrical connection to the common electrode pattern (4). The slit has a width of no less than 0.5 mm for example, particularly no less than 0.8 mm for controlling to provide a proper turnover (R) of the auxiliary electrode layer (6).