A pharmaceutical composition for treating cancer, containing a crRNA and an endonuclease as active ingredients is disclosed. The composition can be customized according to the needs of patients or cell types by specifically treating cancer cells on the basis of specific binding properties of DNA and RNA. The nuclease activity of a CRISPR PLUS system, containing both an endonuclease and an exonuclease can be activated by means of the binding between crRNA and a gene specifically found in cancer cells. Therefore, the cancer treatment effect of the composition is more specific than that of other anti-cancer agents that have been developed up till now.