Orthocresol or a mixture of orthocresol and paracresol and an aldehyde compound or a derivative thereof are subjected to polycondensation in a solvent selected from an aliphatic alcohol having 3 to 12 carbon atoms, a glycol ether having 3 to 6 carbon atoms, benzyl alcohol and an aliphatic carboxylic acid having 2 to 6 carbon atoms, in the presence of an acid catalyst, to obtain linear and high molecular weight orthocresol-novolak resins or orthocresol/paracresol random copolymerized novolak resins which have a high softening point of 145.degree. C. or higher and a high number average molecular weight of 1500 or more and which are highly soluble in alcohols and ketones and are highly compatible with resins.