The invention provides a process which enables, in preparation of acrolein by catalytic gas-phase oxidation of propylene in the presence of molecular oxygen or a molecular oxygen-containing gas or in preparation of acrylic acid by catalytic gas-phase oxidation of acrolein in the presence of molecular oxygen or a molecular oxygen-containing gas, using single kind of atalyst, to suppress occurrence of localized extraordinarily high temperature spots (hot spots) in the catalyst layer and can stably maintain high acrolein or acrylic acid yield for a long time. The process is characterized by use of an oxide catalyst containing molybdenum as an essential component and having relative standard deviation of its particle size in a range of 0.02 to 0.20.