Complex oxide catalysts represented by the formula,(in which A is Ni or Co; B is Na, K, Rb, Cs or Tl; C is an
alkaline earth metal; D is P, Te, Sb, Sn, Ce, Pb, Nb, Mn, As, B or Zn; E is Si, Al, Ti or Zr; and where a is 12, 0<=b<=10, 0<c<=10, 0<d<=10, 2<=e<=15, 0<f<=10, 0<=g<=10, 0<=h<=4 and 0<=i<=30)are provided. The catalysts are characterized in that the
molar ratio of the total
nitrate anions to the
molybdenum at the time of catalyst preparation is more than 1 but not more than 1.8. When used in the reaction for producing (
meth)
acrolein and (
meth)
acrylic acid by vapor-phase oxidation of at least a compound selected from propylene,
isobutylene, t-
butanol and methyl-t-butyl
ether, the catalysts exhibit excellent activity and selectivity and maintain stable performance over prolonged period.