In a head drum assembly mounting structure on a deck of a tape recorder, a plurality of connecting holes and positioning holes are formed in a drum boss provided at a lower portion of a fixing drum, and a plurality of coupling holes and positioning pins are formed in a drum base which is slant disposed on a main base. By using the positioning holes and the positioning pins as positioning means which are connected to each other in complementary fashion, the coupling holes of the drum base and the connecting holes of the drum boss are aligned with each other, and by fastening a screw into such aligned coupling holes and the connecting holes, the head drum assembly can be directly coupled to the main base while being slant disposed with respect to the main base. Accordingly, any possible damage to a lead line is prevented, and assembly efficiency is improved, while the number of parts and the manufacturing cost are reduced.