A straight tube LED lamp includes: a straight tube in which a plurality of light emitting diodes is housed; a first cap for forming a power feeding connection to the plurality of light emitting diodes, provided on one axial direction end side of the straight tube; and a second cap for grounding, provided on another axial direction end side of the straight tube. A first terminal for forming an electrical connection to a power feeding terminal of a first lamp socket is provided in the first cap. A second terminal for forming an electrical connection to a grounding terminal of a second lamp socket is provided in the second cap.

A holder having a mistake-proof structure includes a holder body, two conductive plates, a rotatable fixing element and an installation fixing element. The holder body has a top hole, a central post, and a central recess. The central post is disposed in the central recess. The conductive plates are disposed in the holder body. The rotational fixing element is rotatably connected to the central post and has an extending portion and two through holes disposed opposite each other. The rotational fixing element is rotatable to align the through holes with the top hole. The installation fixing element passes through the top hole and the through holes.