The present invention provides a novel antitumor agent that acts through a novel mechanism. The novel antitumor agent contains as an active ingredient a substance capable of inhibiting a cell cycle-dependent Rho GTPase activating protein (RhoGAP). The RhoGAP is cell cycle-dependent and plays an important role in a process through which cancer cells acquire invasive capacity and/or metastatic capacity. The invasion and/or metastasis of cancer cells can be controlled by targeting the RhoGAP. Examples of the substance capable of inhibiting a cell cycle-dependent RhoGAP include an antisense oligonucleotide against a gene encoding the RhoGAP and an oligonucleotide that induces RNA interference of the gene. As the oligonucleotide, one containing an artificial nucleic acid such as BNA is preferred because of its excellent stability. The present invention also provides a screening method including selecting an antitumor agent capable of suppressing cancer cell invasion and/or metastasis through a novel mechanism. The screening method is a screening method for a novel antitumor agent, including selecting a substance capable of inhibiting a cell cycle-dependent Rho GTPase activating protein.
