An incubator having a cassette tray accommodation space which allows for a cassette tray to be readily stored in or taken out of the space through one of a plurality of tray loading or unloading ports. The cassette tray accommodation space includes at least two of the following: a front tray loading or unloading port, a rear tray loading or unloading port, a left tray loading or unloading port, and a right tray loading or unloading port.