This disclosure provides a method for producing a hydrofluoroolefin (HFO) or fluoroolefin (FO), which is a target compound, with high selectivity, without requiring a high temperature and a catalyst in a synthesis process of the target compound. Specifically, the present disclosure provides a method for producing a hydrofluoroolefin or fluoroolefin having two, three, or four carbon atoms, comprising irradiating at least one starting material compound selected from the group consisting of hydrofluorocarbons having one or two carbon atoms and hydrofluoroolefin As having two or three carbon atoms with ionizing radiation and/or an ultraviolet ray having a wavelength of 300 nm or less, with the proviso that when the at least one starting material compound is the hydrofluoroolefin A, the resulting hydrofluoroolefin is different from the hydrofluoroolefin A.