A Ras, Raf, MEK, ERK or RSK inhibitor, namely a P-glycoprotein expression inhibitor or a BCRP expression inhibitor, can be screened by utilizing the MAPK signaling activity as an indicator. It becomes possible to provide an anticancer agent which is reduced in resistance acquisition, and also provide an agent for preventing the resistance against an anticancer agent, which can enhance the therapeutic effect of the anticancer agent against cancer.