TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Catalyst: The zeolite catalyst is of proton type. The additional amount of zeolite seed crystal is 1-60 mass% of alkaline earth metal containing MFI structure zeolite catalyst amount synthesized without adding zeolite seed crystal. The zeolite seed crystal has MFI structure. EXAMPLE - Zeolite raw material liquid consisting of hydrated aluminum nitrate (Al(NO3)3.9H2O) (in g) (9.5) and calcium methyl acetate (Ca(CH3COO)2.H2O (10.92) was dissolved in water (750). Cataloid silicon-30 water glass (500) dissolved in water (333), 6 mass% sodium hydroxide aqueous solution (177.5), 21.3 mass% tetrapropyl ammonium bromide aqueous solution (317.6) and zeolite Zeolyst (15) (10 mass% of zeolite catalyst amount synthesized without adding zeolite seed crystal) having silicon/aluminum atomic ratio of 70 and average particle diameter of 0.5 microns, was added into obtained liquid with stirring and the aqueous gel mixture was obtained. Subsequently, aqueous base gel mixture was added to 3 L autoclave container and hydro thermally synthesized by stirring at 160degreesC for 18 hours under pressure. The white solid product was filtered, water washed, dried at 720degreesC for 5 hours and baked at 520degreesC in air for 10 hours. The baked product was immersed to 0.6 N hydrochloric acid, stirred at room temperature for 24 hours and zeolite of proton type was obtained. The product was dried at 120degreesC after filtration-water wash for 5 hours, baked at 520degreesC for 10 hours in air, and proton type alkaline earth metal containing MFI structure zeolite catalyst was obtained. The atomic ratio of silicon and aluminum, and that of calcium and aluminum in the raw material preparation were 100 and 2.5, respectively. The atomic ratio of silicon and aluminum, and that of calcium and aluminum in the zeolite catalyst were 100 and 3.7 respectively. The zeolite catalyst had average particle diameter of 1.5 microns, specific surface area of 320 m2/g, catalyst life of 610 g-DME/g-catalyst and 2-4C olefin yield of 79 mass%.