An antitumor agent containing, in combination, at least one kind of antitumor agent selected from the group consisting of an antitumor agent that forms a cross-link with DNA and shows an antitumor effect, an antimetabolite antitumor agent and a taxane antitumor agent, and a histone deacetylase inhibitor. According to the present invention, an antitumor agent causing reduced side effects and having a superior antitumor activity can be provided.
