To provide a supplementary device for getting on and off a dental treatment seat which helps a patient riding a wheelchair to get on and off a dental treatment seat when the patient gets on and off the dental treatment seat.

This getting on and off supplementary device 30 is arranged in front of the dental treatment seat 10 to help the patient riding the wheelchair 20 to get on and off the dental treatment seat 10. The supplementary device 30 has a rotating disk 33 and a pair of armrests 341 and 342 erected opposingly to each other in parallel at a prescribed interval on the rotating disk 33. The armrests can be rotated back and forth keeping the parallel state. In addition, armrest arms 351 and 352 can also be extended and contracted upward and downward.