nearest position to the oral cavity of the patient short tube of the light polymerization means a can be used and the operator can practice easily without having stress the neighborhood id may be around the headrest 1 a at the back or side of the backrest 1 b or may be at the back of the projecting part 1 e as the operator can use the instrument a connected to the instrument connection port 2 other than the instrument 2 around the shoulder of the back of the backrest 1 b and the instrument 4 a held at the tray table 4 while sitting in the operator 39 s chair 5 a the kinds and number of the usable instruments are increased so that extensive care can be executed further the instrument connection port 2 can be made a multi connection which can connect any necessary dental treatment instruments in such a case if the number of the connection port 2 capable of providing at the back or side of the headrest 1 a or at its neighborhood 1 d is limited instruments more than the number of the ports can be used so that extensive care can be executed instruments connected to the instrument connection port 2 are.