When used together with antitumor agents, compounds having an Rho kinase inhibitory activity such as (R)-(+)-N-(4-pyridyl)-4-(1-aminoethyl)benzamide can potentiate the antitumor effect of the antitumor agents. Thus, these compounds are useful as antitumor effect potentiators. In particular, the combined use of these compounds having an Rho kinase inhibitory activity with antitumor agents makes it possible to reduce the dose of the antitumor agents, thereby presenting a sufficient effect and/or relieving side effects.