Excavated bronze objects, depending on how long they have been in the ground and under what conditions, are generally corroded externally as well as having intricately corroded layers inside. To date, our group has performed metallurgical investigations on 18 ancient bronze mirrors and confirmed that pure copper lumps and several varieties of unidentified corrosion products have formed on the surface of the mirrors and in the corroded layers. Accordingly, we performed investigations to identify the corrosion products using an electron probe micro analyzer (EPMA), a scanning electron microscope (SEM) equipped with an energy dispersive spectroscope (EDS), a micro-X-ray diffraction analyzer (μ-XRD), and an X-ray photoelectron spectroscope (XPS), and discussed the possibility that bacteria play a role in inducing corrosion in ancient bronze mirrors. The results are as follows; (1) Amorphous SiO₂ had formed in the outermost corroded layer I. (2) Cu₂O (cuprite) and Cu(OH)₂ had formed in the layers II and III, which comprise the first two internal layers adjacent to layer I. (3) Cu–Sn–Pb-complex oxide had formed in layer IV, the next internal layer. Small amounts of PbSO₄ and an Sn-organic compound were also detected. (4) We observed that Cu₂S (chalcocite) forms like mold on the polished surface of the corroded layers, especially notable on layer IV, after being mechanically polished and left to stand for about one month. Cu₂S is presumed to be the resultant of sulfate reducing bacteria. (5) Numerous pure copper lumps appeared in layers or particles and tended to form in the vicinity of layer IV. (6) The evidence from these analyses suggest that microorganisms could have been the cause of the corrosion found in ancient bronze mirrors excavated from the ground.