A condensed phosphoric ester having a very low content of isopropenyl phenyl diphenyl phosphate, which reduces heat resistance and causes the contamination of molds during the molding when the condensed phosphoric ester is used as an additive for a resin, is provided by a process including a first step of reacting a phosphorus oxytrihalide with a bisphenol A derivative at a specific ratio to prepare a phosphorohalidate; a second step of removing unreacted phosphorus oxytrihalide; a third step of reacting the reaction product obtained in the second step with a monophenol-based compound at a temperature equal to or lower than 120°C; and a fourth step of reacting the phosphorohalidate with the monophenol-based compound at a temperature equal to or higher than 120°C.