(EN) 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 anti-cancer agent which is reduced in resistance acquisition, and also provide an agent for preventing the resistance against an anti-cancer agent, which can enhance the therapeutic effect of the anti-cancer agent against cancer.