PURPOSE: To obtain the titled compound having the pharmacological action in high yield, with the reduced cost of the raw material and the load on the treatment of waste acids, by the selective ring closing reaction of a 5-(1-carboxyethyl)-2- phenylthiophenylacetic acid with polyphosphoric acid and an organic solvent.

CONSTITUTION: A compound of formulaIis subjected to the ring closing reaction with polyphosphoric acid and an organic solvent, e.g. methylene chloride or 1,2-dichloroethane, to give the aimed compound of formula II. The reaction is carried out by using the polyphsphoric acid in an amount of 2W10 times, preferable 4W6 times that of the compound of formulaI, and the organic solvent in a volume of 2W10 times, preferably 4W6 times, that of the compound of formula I. The reaction temperature is preferably 40W60°C, and the temperature of 70°C or above is disadvantageous due to the occurrence of side reactions. The reaction can be carried out without using a large excess of a condensing agent, and the ring closing reaction of the compound of formulaIhaving two carboxylic acid groups can be carried out in high yield.