Method for producing unsaturated acid and/or unsaturated acid ester

A method for producing an unsaturated acid and/or an unsaturated acid ester, comprising a process A of reacting a compound (1) represented by the following formula (1) at a temperature of 0° C. to 350° C. in the presence of a Brønsted acid catalyst and/or a Lewis acid catalyst, to prepare a compound (2) represented by the following formula (2);

• (wherein each of R1, R2 and R4 independently represents a hydrogen atom, a deuterium atom or an alkyl group which has carbon number of 1 to 3 and may be substituted with a halogen atom and/or a deuterium atom;
• each of R3 and R5 independently represents a hydrogen atom or a deuterium atom;
• R6 represents a hydrogen atom, a deuterium atom, or an alkyl group or an aryl group which has carbon number of 1 to 8 and may be substituted with a halogen atom and/or a deuterium atom;
• and X represents a chlorine atom, a fluorine atom, a bromine atom, or an iodine atom).