This invention relates to a method to improve 1,1,3-trichloropropene andor 3,3,3-trichloropropene selectivity in the dehydrochlorination of 1,1,1,3-tetrachloropropane. In a reset period Tr a reset voltage is applied to each pixel electrode. The magnetic flux detector also has a search coil provided in a magnetic path for the magnetic flux, and a magnetic flux calculator, which determines the magnitude of the magnetic flux by measuring an electromotive force induced in the search coil by a change in the magnetic flux.