Even when a membrane sheet contains an inorganic material, the membrane sheet and a support plate are properly deposited by a laser emission. In a manufacturing method of a flat membrane element lapping a membrane sheet performing solid-liquid separation on a support plate supporting the membrane sheet and emitting a laser to a mutual joint portion to deposit the membrane sheet and the support plate, when the membrane sheet contains an inorganic material, the support plate is formed by a material having laser penetration and the laser is emitted from the support plate side to the joint portion.