Abstract: A proliferation inhibitor for canine cancer cells contains, as an active ingredient, 1-(3-((6,7-dimethoxyquinazolin-4-yl)oxy)phenyl)-3-(5-(1,1,1-trifluoro-2-methylpropan-2-yl)isooxazol-3-yl)urea or CEP-32496, and a method for suppressing proliferation of canine cancer cells. The cancer cells are caused by BRAF gene mutation. The canine cancer is canine transitional epithelial cancer, canine prostate cancer, canine malignant melanoma, or canine peripheral schwannoma.