Factors that promote the aggressiveness of squamous cell carcinoma of the breast are not well understood. To examine the involvement of cell motility and the mechanism of this behavior, a squamous cell carcinoma cell line of the breast (HBC9) was established from a metastatic lymph node of a Japanese woman. HBC9 expressed epidermal growth factor receptor (EGFR), but was negative for Her2 or Her3.The invasive ability of HBC9 was compared with that of four breast ductal carcinoma cell lines by Matrigel invasion assay. EGF stimulation induced the formation of surface protrusions and cell migration in HBC9 cells, and significantly increased the number of cells migrating through the Matrigel. The invasive ability of HBC9 was compared with other cell lines of breast carcinoma; it was much greater than that of MCF-7, BT474, or HBC5, but did not differ significantly from that of MDA-MB-231. Observation of the surface protrusions of HBC9 by confocal laser microscopy revealed co-localization of Arp2 and N-WASP with actin polymerization, detected by visualization with phalloidin, indicating that the protrusions induced by EGF were invadopodia. In HBC9 cells, cortactin also co-localized with the N-WASP/Arp2/3 complex in the protrusions. Immunohistochemistry of 12 cases of squamous cell carcinoma of the breast revealed expression of cortactin and EGFR in all of them, and this was confirmed by western blotting in two cases. These results suggest that EGF-dependent enhancement of cell motility by formation of invadopodia associated with cortactin is a cause of the clinical aggressiveness of squamous cell carcinoma of the breast.