PURPOSE: To produce synthetic zeolite crystals by increasing the apparent surface area of ZSM-5 zeolite crystals which are originally spherical or platy.

CONSTITUTION: An aq. mixture contg. tetraalkoxysilane having lower alkoxy groups as a silica source, an alumina source, an alkali metal source and a tetrapropylammonium salt as a crystallizing agent is sealed in an autoclave and held at a hydrothermal synthesis temp. of 160-200°C to produce the objective square pillar-shaped synthetic ZSM-5 zeolite crystals.