It is an object to provide a novel oxide catalyst for producing an unsaturated acid or unsaturated nitrile by which reaction results are good and a high yield can be stably maintained for a prolonged period of time, a process for producing the oxide catalyst, and a process for producing an unsaturated acid or unsaturated nitrile using the oxide catalyst. According to the present invention, there is provided an oxide catalyst represented by following compositional formula (1): Mo1VaSbbNbcMndWeYfOn  (1) wherein Y represents at least one element selected from alkaline earth metals and rare earth metals; a, b, c, d, e, f, and n each represents an atomic ratio based on one atom of Mo; 0.1≦a≦1, 0.01≦b≦1, 0.01≦c≦1, 0≦d≦0.1, 0≦e≦0.1, 0