PURPOSE: To improve quality by removing Al2O3 by a water heat treatment of zeolite to increase its Si/Al ratio and selectively extracting formed amorphous phase Al2O3 thereby forming a zeolite product holding a framework crystallization degree.

CONSTITUTION: The natural or synthetic crystalline aluminasilicate zeolite is subjected to the water heat treatment for about 1 to 24 hours at about 427 to 816°C in an atmosphere contg. at least 2% of water, by which the Al2O3 is selectively removed from crystal lattice and the molar ratio of Si/Al is increased to about 8 to 50 and in addition, the non-framework amorphous phase Al2O3 is formed. Next, org. acids, such as trichloroacetic acids, expressed by formula I to form III (X is 1 to 3; R is phenyl, 1 to 10C alkyl) are added to the treated zeolite, by which the amorphous phase Al2O3 is selectively extracted. The high silica crystalline zeolite contg. the non-framework Al2O3 at about <5wt.% and substantially maintaining the framework crystallization degree is thus obtd.