In head and neck clinical oncology, recurrent cancer after initial irradiation therapy is no longer sensitive to irradiation. To explore the irradiation resistance in head and neck squamous cell carcinoma, a human cell line, KB, derived from the floor of the oral cavity was used. The participation of the Fas-mediated apoptotic pathway was suggested by the upregulation of the surface Fas molecule, the reduction of the apoptotic cell fraction after inhibition of caspase 8 which is a Fas-related initiator caspase, and the changes in Fas-related genes after irradiation. Therefore, it is suggested that disruption of the Fas-mediated apoptotic pathway participates in the acquisition of irradiation-resistance in HNSCC.