The present invention addresses the problem of providing an antitumor agent and an antitumor kit having a superior antitumor effect in comparison to gemcitabine, an antitumor platinum complex and a combination therapy thereof, and an antitumor effect potentiator. Provided is an antitumor agent that comprises an antitumor platinum complex and 1-(2-deoxy-2-fluoro-4-thio-β-D-arabinofuranosyl)cytosine or a salt thereof.