Abstract: As an improvement in the process for preparing unsaturated aldehydes and unsaturated carboxylic acids through vapor-phase catalytic oxidation of at least one starting compound selected from propylene, isobutylene, t-butanol and methyl-t-butyl ether with molecular oxygen or a molecular oxygen-containing gas, using a fixed bed shell-and-tube type reactor which is filled with shaped catalysts, a process capable of effectively inhibiting occurrence of hot spots or excessive heat generation at the hot spots and producing unsaturated aldehydes and unsaturated carboxylic acids at high yields is provided.