The present invention relates to a method for producing an unsaturated-acid ester or an unsaturated acid, containing a step of reacting a compound (1) represented by the following formula (1) with a compound represented by the following formula (2) (excluding the compound (1)) in the presence of a Lewis acid catalyst at a temperature of the boiling point of the compound (1) or higher and 350° C. or lower, thereby obtaining products including a compound represented by the following formula (3): (in formula (1) and formula (3), R1, R2 and R4 each independently may be hydrogen atom or an alkyl group, R3 and R5 each independently are hydrogen atom or a deuterium atom, and X is a halogen atom; in formula (2) and formula (3), R6 may be hydrogen atom, an alkyl group or an aryl group, and R7 is hydrogen atom or a deuterium atom).