The object of the present invention is to provide a process in which, when the unsaturated carboxylic acid is produced, or when the unsaturated aldehyde and/or the unsaturated carboxylic acid are produced by carrying out the catalytic gas phase oxidation reaction by using the fixed-bed multitubular reactor which is packed with the molybdenum-containing catalyst, the reaction can be continued for a long time while a high yield is maintained, regardless of where the hot spot portion occurs and also even if the concentration of the raw gas is high. To solve this object, the present invention production process for an unsaturated carboxylic acid is characterized in: a particulate catalysts including, as catalytic components, an oxide and/or a complex oxide including Mo and V, or Mo, Fe and Bi as essential components, and having a hole are used; and a catalyst-packed layer of each reaction tube of the fixed-bed multitubular reactor is divided into at least two reaction zones in a tubular axial direction, wherein the packing of the catalysts is such that the diameters of the holes of the catalysts differ between at least two of the reaction zones.