To provide a novel process for producing p-dichlorobenzene satisfying both of high selectivity of p-dichlorobenzene and high conversion of chlorine at the same time.A process for continuously producing p-dichlorobenzene, which is a process for producing p-dichlorobenzene by a nuclear chlorination reaction of benzene and/or chlorobenzene with chlorine, in the presence of a Lewis acid catalyst and a phenothiazine analogue compound, said process comprising; employing a reactor having a first supply route for continuously supplying a mixed solution of benzene and/or chlorobenzene and a Lewis acid catalyst to a reactor, a second supply route for continuously supplying a mixed solution of benzene and/or chlorobenzene and a phenothiazine analogue compound to the above reactor, and a third supply route for continuously supplying chlorine to the above reactor; and supplying these raw materials for the reaction by opening the above first supply route and third supply route in random order at the initiation of reaction, and then opening the above second supply route.