When used concurrently with an antitumor agent, a compound having a Rho kinase inhibitory activity, such as (R)-(+)-N-(4-pyridyl)-4-(1-aminoethyl)benzamide, reinforces an antitumor effect of the antitumor agent and is useful as an antitumor effect potentiator. Particularly, a compound having a Rho kinase inhibitory activity used with an antitumor agent in combination can reduce the dose of the antitumor agent, which in turn affords provision of a sufficient effect and/or reduction of side effects.