Provided are: a method for producing a cured product, including a curable composition preparing step of preparing a curable composition containing a polyiso(thio)cyanate compound, a polythiol compound, and a curing catalyst having a water content of 5000 ppm or less on a mass basis, and a curing step of curing the curable composition to obtain a cured product; a cured product obtained by curing a curable composition containing a polyiso(thio)cyanate compound, a polythiol compound, and a curing catalyst having a water content of 5000 ppm or less on a mass basis; and an eyeglass lens substrate including the cured product.