PURPOSE: To make it possible to easily and completely disinfect a water circuit and air circuit in an instrument hose and to prevent misoperation, etc., by adopting the constitution that the disinfecting chemical in a disinfecting liquid chamber is released from an instrument through the air circuit and/or the air circuit.

CONSTITUTION: The bottom end face of the instrument holder 3 is provided with a connecting terminal 11 for freely attachably and detachably connecting the terminal 7 of the instrument holder 6. A disinfecting container 20 is so constituted that the one end, between its terminals, is freely attachably and detachably connected to the terminal 11 on a dental treatment unit 10 side and the other end to the terminal 7 of the instrument hose 6. Namely, the water circuit and air circuit on the dental treatment unit 10 side are connected to the side connected with the dental treatment unit 10 of the disinfecting container 20 and the water circuit and air circuit on the instrument hose 6 side is connected to the other side.