PURPOSE: To obtain a polymer for an SiN ceramic precursor or the like in which its viscosity can be easily regulated by reacting NH3 with a halogenated Si compd. to a prepare a silazane ammonolysis product and reacting the silazane ammonolysis product with an isocyanate compd. or the like.

CONSTITUTION: Ammonia is reacted with halogenated Si compds. (at least one of the halogenated Si compds. contg. an Si-H bond), such as formula I [wherein R represents H, a 1 to 6C (substd.) alkyl, an aryl, a 1 to 6C alkenyl or the like; and X represents Cl, Br or I], formula II (wherein R' has the same meaning as R), and a mixture thereof to prepare a silazane ammonolysis product, which is then reacted with a compd., such as an isocyanate, an isothiocyanate, a ketene, a thioketene, a carbodiimide or carbon disulfide, in an amt. of about 0.1 to 30 wt.% based on the wt. of the ammonolysis product to prepare the objective polysilazane-added polymer.