There is disclosed a method of producing .alpha.,.beta.-unsaturated carboxylic acid of formula (3) ##STR00001## wherein R.sup.1 denotes a hydrogen atom, an alkyl group, an aryl group, an aralkyl group, an alkenyl group or an alkynyl group, which method comprises reacting a carboxylic acid of formula (1) ##STR00002## wherein R.sup.1 denotes the same as the above, or carboxylic anhydride of formula (2) ##STR00003## wherein R.sup.1 denotes the same as the above, with a methylenating agent in a presence of a catalyst containing metal of Group 13 of the Periodic Table of Elements in the amount of not more than 5 weight %.
