A method of installing a large lightweight lighting fixture includes: providing connector-terminated wiring in a space above a ceiling, the connector-terminated wiring being electrically connecting to the large lightweight lighting fixture, the ceiling comprising a ceiling material, the lighting fixture mounted to the ceiling without being fixed by a suspension member; connecting the connector-terminated wiring and a power supply adapter by using a connector of the connector-terminated wiring via an opening provided in the ceiling material; electrically connecting the lighting fixture and the power supply adapter while the lighting fixture is located in a room space below the formed ceiling; and mounting the lighting fixture to the ceiling material by passing a fastener through the opening of the ceiling material, wherein the fastener is included in (i) the lighting fixture, or (ii) the power supply adapter.