We constructed a replication-competent oncolytic adenovirus, OBP-301 (Telomelysin), in which human telomerase reverse transcriptase (hTERT) promoter drives E1 genes. OBP-301 is currently being used in a phase-I clinical trial for various types of tumors. Under such conditions, anti-adenoviral agents should be available for safety use against OBP-301 since any adenoviral viremia could cause severe adverse effects. Cidofovir (CDV) is an acyclic nucleoside phosphonate that has a broad antiviral activity against DNA viruses. Here, we examined the antiviral effects of CDV against OBP-301. The in vitro cytopathic effects of OBP-301 were suppressed by CDV. Moreover, CDV decreased the adenoviral E1A gene copy number after OBP-301 infection. These results suggest that CDV is a potentially useful antiviral agent for OBP-301.