The present invention relates to a method of treating cancer by co-administration of an effective amount of 1-(2-methoxyethyl)-2-methyl-4,9-dioxo-3-(pyrazin-2-ylmethyl)-4,9-dihydro-- 1H-naphtho[2,3-d]imidazol-3-ium bromide and an effective amount of one or more anticancer agents selected from the group consisting of carboplatin, cisplatin, paclitaxel, vinorelbine, gemcitabine, irinotecan, docetaxel, doxorubicin, dacarbazine and rituximab, or a retuximab-containing combination therapy selected from R-ICE and R-DHAP. The treatment method of the present invention is useful for the treatment for all solid tumors and lymphomas, preferably skin cancer, bladder cancer, breast cancer, uterine cancer, ovary cancer, prostate cancer, lung cancer, colon cancer, pancreas cancer, renal cancer, gastric cancer and the like. Particularly, they are expected as therapeutic agents for tumor types which show resistance against existing anticancer agents.