It is an object of the invention to provide a method for processing a very large number of fish eggs for gene injection and so on, which employs a simple system with high performance and a reasonable construction expense. It is another object to provide the method having an water tank apparatus capable of producing a large number of fish eggs continuously.A large number of water tank groups having an independent drainage passage each is lighted up in turn with a predetermined interval. An egg-collecting case with a net bottom is set in the drainage passage connecting to the water tank group to which the lighting is started. The water tank group consists of a plurality of tank unit accommodating fishes each. Each book-shaped tank unit has a rectangular-shaped upper portion and a cone-shaped lower portion. Rotating water stream is formed in each tank unit.