A method for efficiently obtaining trans-1-halo-2-fluoroethylene (E-isomer) and/or cis-1-halo-2-fluoroethylene (Z-isomer) is provided. The method for producing trans-1-halo-2-fluoroethylene (E-isomer) and/or cis-1-halo-2-fluoroethylene (Z-isomer) includes supplying a composition containing trans-1-halo-2-fluoroethylene (E-isomer) and/or cis-1-halo-2-fluoroethylene (Z-isomer) to a reactor to perform isomerization between the trans-1-halo-2-fluoroethylene (E-isomer) and the cis-1-halo-2-fluoroethylene (Z-isomer) in a liquid phase by performing light irradiation in the presence of a photosensitizer.