In reacting an anthranilic acid derivative represented by the general formula (1), especially the anthranilic acid derivative selected from anthranilic acid, anthranilamide and anthranilate, with formamide, the reaction is attained under the condition of coexistence of acetic acid and a base as a catalyst in the reaction liquid, and it has made it possible to produce a quinazolin-4-one derivative represented by the general formula (2) and useful as a material for medicine intermediates, at high yield with no side production.