To fix a swing arm to a prescribed position from which an instrument is pulled out during an operation, and to release the fixing by returning the instrument to an instrument holder after the operation to automatically return the swing arm to a position in non-use.

This operation mechanism for a dental instrument hose includes: a spring 12 stretched such that the swing arm 10 holding the instrument hose is turned rearward; and a motor 30 turning a turning shaft of the swing arm 10 such that the swing arm turns forward. When the instrument is picked up from the instrument holder, the motor 30 is driven to turn the swing arm 10 forward, and an electromagnet 33 is energized. When the swing arm 10 reaches a forward prescribed position b, the swing arm is fixed to the prescribed position b by the electromagnet 33. When returning the instrument to the instrument holder, the energizing of the electromagnet is released, and the swing arm 10 is pulled to a rearward prescribed position a by tensile force of the spring 12, and is fixed to the prescribed position b.