A laboratory animal breeding device comprises breeding cages, each including a cage body with a part of the cage body being opened and a removable cover for hermetically sealing the opening in the cage body. An air supply device for ventilation inside the breeding cage is attached to the cover of the breeding cage. The breeding cage includes an air exhaust device that is formed of a fan filter unit comprising a fan device and a first filter. The air exhaust device includes an air exhaust opening formed in the breeding cage and a second filter for cleaning air in the breeding cage passing through the air exhaust opening. Outside air is taken into the breeding cage while being cleaned by the fan filter unit, and contaminated air in the breeding cage is exhausted from the air exhaust opening through the second filter to outside of the breeding cage.