An object of the present invention is to provide an enhancing agent for effect of anticancer agent for achieving an excellent therapeutic effect on cancer. The enhancing agent for effect of anticancer agent according to the present invention which is a solving means therefor is characterized in that a nitric oxide donor is an effective ingredient. In accordance with the present invention, an excellent therapeutic effect is able to be achieved on non-small cell lung cancer which is still in such a state that no effective therapeutic method has been established yet for a progressive cancer which is not operable and is one of cancers where chemotherapy is most difficult to apply.