To reuse support plate and to fix a membrane sheet by the same method both in a new product manufacturing and in regeneration. In a flat membrane element including a deposit fixed portion formed by lapping a membrane sheet performing solid-liquid separation onto a support plate supporting the membrane sheet and by emitting a laser to a circumferential edge of the membrane sheet to deposit fix the member sheet onto the support plate, and making an inside of the membrane sheet surrounded by the deposit fixed portion be a solid-liquid separating area; an embrocation having laser absorption property is applied on the surface of the membrane sheet of the deposit fixed portion. At the time of regeneration, such a portion of the membrane sheet that corresponds to the solid-liquid separating area is cut off and, after that, a new membrane sheet is lapped on the embrocation and the new membrane sheet and the embrocation are deposit fixed by a laser.