Disclosed is an antitumor agent which uses a compound represented by formula (I) or a pharmacologically acceptable salt thereof in combination with a compound represented by formula (II) or a pharmacologically acceptable salt thereof. The antitumor agent exhibits remarkable antitumor effect in comparison to the cases where each one of the above-mentioned compounds is used alone. In addition, the antitumor agent exhibits antitumor effect on various kinds of cancer. (In the formulae, R