To provide a supporting structure for an illuminator for an aquarium having a small number of movable parts and a simple structure and readily attachable to the existing aquarium.

This supporting structure 10 comprises engaging attaching parts 11, axial supporting frame parts 12 extending from the rear of the engaging attaching parts 11 to the upper side, bending at nearly right angles in the course thereof and extending to the front, axial connection fixing parts 13 connected to the vicinity of the tips of the supporting frame parts 12 and extending to the inside in a lateral part and a tubular connection supporting part 14 into which parts to the tips of the connecting fixed part 13 are inserted. The engaging attaching parts 11, the supporting frame parts 12 and the connection fixing parts 13 are mutually joined to compose integral supporting frame members. The supporting structure 10 has a structure in which a pair of supporting frame members of a left and right symmetrical form are connected with the connection supporting part 14.