The purpose of the present invention is to discaver a new factor which influences the expression level of a cancer promoting factor, and provide an expression inhibitor of cancer promoting factor and developing tool thereof based on the new factor, and provide a diagnostic agent and a diagnostic method of cancer. The solution of the present invention is an expression inhibitor of cancer promoting factor comprising at least one selected from the group consisting of RBMS expression inhibitors and RBMS functional inhibitors, a screening method by using the expression or the function of RBMS as an index, and an expression cassette useful in the method, and a cancer diagnostic agent comprising a RBMS gene expression product detecting agent, and a cancer detetcting method by using RBMS gene expression level as an index.
