PURPOSE: To increase the mechanical strength of the title catalyst and to improve the stability by hydrothermally synthesizing a molybdenum compd. and an iron compd. at ≥100°C under pressure to form iron molybdate.

CONSTITUTION: A molybdenum compd. and an iron compd. are hydrothermally synthesized at about 100W200°C under pressure to form iron molybdate. The iron molybdate is washed with water, and then washed, as required, by using acidic water or hot water, and a solid consisting of the crystal of iron molybdate is obtained. The iron molybdate as such or the molded material can be used as catalyst. Besides, a carrier component inert to the reaction is added to the iron molybdate, and the mixture is molded to an appropriate particle diameter. When the obtained material is used for the reaction, the reaction can be appropriately controlled, the mechanical strength is enhanced, and the material is advantageous from the economical point of view.