Incubation device (10) has a structure wherein an incubation tank (11) and a separation tank (12) are put into communication with each other at the lowermost parts via a narrow communicating port (13). A side opening (14) is formed in the separation tank (12). The wall of the lower part of an upper housing part (12a) and of the entirety of a lower housing part (12b) of the separation tank (12) are formed from a colored, light-shielding material OP. Sea water (D) is housed in incubation tank (11) and the lower part of separation tank (12), freshwater (E) is housed in the upper part of the separation tank (12), and an interface (F) between the sea water (D) and the freshwater (E) is formed inside the separation tank (12). Brine shrimp larvae (2) hatch in the incubation tank (11) and upon moving into the separation tank (12), move from the light-shielded lower part towards the unshielded upper part and eventually swim outside into an aquarium tank from the side opening (14).