The invention provides a catalyst for producing acrylic acid at high yield for a long time, in a method for producing acrylic acid by catalytic gas phase oxidation of propane and/or acrolein in the presence of molecular oxygen or a molecular oxygen-containing gas. This catalyst comprises a complex oxide containing molybdenum, vanadium and X component (here the X component is at least one element selected from antimony, niobium and tin) as the essential components, and is characterized in that its main peak as measured by X-ray diffractiometry using Kα ray of Cu, d=4.00±0.1 angstrom, and in that the particle size of the X component in the catalyst does not exceed 20 μm.