An oxide catalyst for use in the production of unsaturated aldehyde, diolefin, or unsaturated nitrile from olefin and/or alcohol, the oxide catalyst satisfying the following (1) to (3):
- (1) the oxide catalyst comprises molybdenum, bismuth, iron, cobalt, and an element A having an ion radius larger than 0.96 Å (except for potassium, cesium, and rubidium);
- (2) an atomic ratio a of the bismuth to 12 atoms of the molybdenum is 1≦a≦5, an atomic ratio b of the iron to 12 atoms of the molybdenum is 1.5≦b≦6, an atomic ratio c of the element A to 12 atoms of the molybdenum is 1≦c≦5, and an atomic ratio d of the cobalt to 12 atoms of the molybdenum is 1≦d≦8;
- and
- (3) the oxide catalyst comprises a disordered phase consisting of a crystal system comprising the molybdenum, the bismuth, the iron, and the element A.