Process for preparing catalyst

Relates to a catalyst used in the present invention is effectively utilized, the used catalyst containing at least molybdenum, an element A (at least one element selected from phosphorus and arsenic) and one X element (at least one element selected from potassium , rubidium and cesium element), and provides a process for preparing a catalyst which comprises the used catalyst is dispersed in water, to which is added an alkali metal compound and / or ammonia solution, the resulting mixture was then adjusted pH to 6.5 or less to produce a precipitate containing at least a element and molybdenum, and the precipitate was used as a raw material providing element constituting the catalyst.