Abstract: A process for advantageously producing, in a short period of time, a phenolic novolak resin, in which substantially no unreacted aldehyde remains, by using a continuous reactor, while a generation of high order condensation products is prevented. In continuously producing the phenolic novolak resin by reacting at least one of phenols with at least one of aldehydes in the presence of an acidic catalyst by using a continuous reactor which has a long reaction tube: the reaction tube is provided with a heating zone and a temperature control zone; a mixture of raw materials is heated, at the heating zone, to a temperature not less than a temperature, at which a reaction heat is generated; and then the mixture is subjected to a cooling operation at the temperature control zone, while the pressure within the reaction tube is kept to be not lower than a vapor pressure of water.