Method of producing formaldehyde directly from methane

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-087641, filed Mar. 27, 2000, the entire contents of which are incorporated herein by reference. The present invention provides a method of using a silica-supported 12-molybdosilicic acid catalyst consisting of 12-moloybdosilicic acid supported on silica, in which 12-molybdosilicic acid is supported in an amount not smaller than 10 mass % on silica, wherein the reaction system is heated in the presence of the silica-supported 12-molybdosilicic acid catalyst to the operating temperature of the catalyst at a rate not lower than 100° C./min.