A silica-supported catalyst used when producing a corresponding unsaturated nitrile in a vapor-phase catalytic ammoxidation reaction of propane or isobutane, the catalyst including a metal oxide represented by the following formula (1), MoVaNbbXcTdZeOn  (1) (wherein X represents at least one or more elements selected from Sb and Te; T represents at least one or more elements selected from Ti, W, Mn, and Bi; Z represents at least one or more elements selected from La, Ce, Yb, and Y; and a, b, c, d, e, and n are in the range of 0.05≦a≦0.5, 0.01≦b≦0.5, 0.001≦c≦0.5, 0≦d≦1, and 0≦e≦1, respectively, and n denotes a value that satisfies an atomic valence) wherein the silica-supported catalyst has an average pore size of 60 to 120 nm, a total pore volume of 0.15 cm3/g or more, a specific surface area of 5 to 25 m2/g, and a crystallite size of 40 to 250 nm as determined from half width of a (001) peak by X-ray diffraction.