For conservation purposes and to supply rare insects for laboratory use, a system for artificial breeding is crucial. However, in the case of carnivorous freshwater insects such as diving beetles, constant conditions in aquariums are difficult to maintain due to their high rate of food consumption. Furthermore, surface rippling caused by the pumping system for water circulation hinders the respiration of small larvae. We developed a new open aquarium system without water circulation that was successfully applied to the rearing of larvae of diving beetles, Dytiscus sharpi (Wehncke) (Coleoptera: Dytiscidae). In comparison to conventional methods, a high proportion of larvae developed into adult insects. The size of reared adults was almost the same as those of field-collected adults. The new method could be applied to the conservation and breeding of other rare species, such as water beetles and water bugs.