PURPOSE: To enhance mechanical resistance, aging resistance or the like by supporting specific amts. of V3O5 and alkali metal oxide on an Fe2O3-contg. silica carrier having a specific particle density, pore volume or the like, thereby forming a porous catalyst.

CONSTITUTION: The catalyst carrier has an SiO2 content of ≤75 wt.% and an Fe2O3 content of ≤0.90 wt.% and has the particle density of 1.10 to 1.40 g/cm3, the pore volume of 0.20 to 0.70 cm2/g, a surface area of 0.30 to 5 m2/g and an average radius of pores of 600 to 2,200 nm is manufactured. The porous catalyst is produced by supporting 6 to 9 wt.% V2O5 and 8.5 to 12 wt.% Me2O (M is an alkali metal) on the catalyst carrier. The porous catalyst decreased in the catalyst components by use is pulverized down to an average diameter of 1 to 50 μm. These particles are immersed into an aq. soln. contg. a necessary catalyst amt., by which the catalyst is regenerated to the fresh catalyst.