A process for removal of solid organic matters formed and deposited in the reaction system when an unsaturated aldehyde or an unsaturated aidehyde-containing gas is subjected to catalytic gaseous oxidation in the presence of an oxidizing catalyst to prepare an unsaturated carboxylic acid corresponding thereto, which process comprises contacting the solid organic matters with a mixed gas containing at least 3 volume % of molecular oxygen and at least 0.1 volume % of steam at a temperature of 260 DEG to 450 DEG C.
