Catalytic conversion of organic compounds wherein catalysts come into contact with steam is performed efficiently by using a catalyst which comprises an MFI type zeolite having a ratio of SiO.sub.2 /(Al.sub.2 O.sub.3 +Ga.sub.2 O.sub.3) of 20 to 200 in molar ratio, a ratio of Ga.sub.2 O.sub.3 /Al.sub.2 O.sub.3 of 0 to 50 in molar ratio and a ratio of peak intensity of SiOH, I.sub.SiOH, to peak intensity of acidic OH, I.sub.H.sup.+, determined from .sup.1 H-NMR, I.sub.SiOH /I.sub.H.sup.+, of 0 to 0.5 and loses little catalytic activity on exposure to steam.