PURPOSE: To utilize an auxiliary lighting system arranged in the vicinity of a main lighting system which is lit and lit out as a water tank lighting system so as not to produce a harmful effect on living things in the water tank in which the main lighting system is set.

CONSTITUTION: An auxiliary lighting system is set in the vicinity of a main lighting system 22 which is lit and lit out, and comprises a miniature bulb 10c, a photosensor 10b for detecting the brightness in a water tank 1 to be illuminated by the auxiliary lighting system, and a control means for lighting the miniature bulb 10c according to the brightness detected with the photosensor 10b. The control means lights the miniature bulb 10c when the brightness detected by the photosensor 10b is lower than a reference value of the brightness previously set.