This invention is based on the discovery by the inventors that, if a preliminary hydration hardening process and a full-scale hydration hardening process are provided after the molding process, and a process for burning a hardened body of cement at a high temperature is provided between the preliminary and full-scale hardening processes, it is possible to obtain a molded product of cement having a higher mechanical strength than any such product according to any known method including no such burning process.According to this invention, there is provided a method of manufacturing a product of cement having high mechanical strength, which method comprises the steps in sequence of adding water, and if required, glass fibers or other reinforcing material, into a mixture of cement and an aggregate, and kneading the whole completely; molding the kneaded mixture into a desired shape; hardening the molded body preliminarily by hydration; burning the preliminarily hardened body at a high temperature; and hardening the burned body on a full scale by hydration.