A working method for an end portion of a plate member includes heating and pressing the end portion of the plate member formed of a thermoplastic resin in a thickness direction to form a thin portion, folding the thin portion such that a pressed surface is positioned inside to move an outer peripheral end of the thin portion as an end portion on an outer peripheral side of the plate member to the vicinity of a main body portion of the plate member to which an inner end of the thin portion as an end portion on an inner side of the plate member is connected, and pressing the outer peripheral end of the thin portion in the thickness direction to move the outer peripheral end thereof further inward than a surface of the main body portion in the thickness direction.