A lamp including a case assembly, a lamp bottom and a fixing elastomer is provided. The case assembly has a first opening, and the lamp bottom inserts into the first opening to assemble with the case assembly. The lamp bottom has a first surface, a second opening and multiple first positioning structures. The second opening passes through the first surface, and the first positioning structures are located on the first surface. The fixing elastomer is disposed in the case assembly and has a second surface and a pair of second positioning structures suitable for locking with two of the first positioning structures.