Abstract: An anesthesia system includes a main unit (10) for supplying anesthetic gas and providing artificial respiration, an induction chamber (20) for receiving a small animal and for introducing the anesthetic gas, an anesthetic gas treatment section (30) connected to the induction chamber (20) and a breathing circuit tube (50) having an intubation portion (53). The main unit (10) has a first outlet port (10a), a second outlet port (10c), a return port (10b) and a communication passage (17) connecting the return port and the second outlet port. The anesthetic gas exiting through the first outlet port (10a) is conveyed through the breathing circuit tube (50) to return to the return port (10b) and further conveyed through the communication passage (17), the second outlet port (10c) and a supply tube (60) into the induction chamber (20).