The present invention provides 2-piperidone compounds or pharmaceutically acceptable salts thereof, which have a potent activity of inhibiting the proliferation of tumor cells and thus are useful as medicaments, as well as antitumor agents containing these compounds. The 2-piperidone compound is represented by the following formula (I): wherein R1 represents —(CH2)nR1a {wherein n is an integer of from 0 to 5, and R1a represents amino, lower alkylamino, di(lower alkyl)amino, substituted or unsubstituted aryl, or a substituted or unsubstituted heterocyclic group}, and R2 and R3 independently represent lower alkyl which may be substituted by lower alkoxycarboyl; lower alkenyl, aralkyl or lower alkynyl which may be substituted by substituted or unsubstituted aryl or a substituted or unsubstituted heterocyclic group; substituted or unsubstituted aryl; or a substituted or unsubstituted heterocyclic group.