PURPOSE: To prevent damage to an assistant's instrument holder and other members by causing a safety switch to operate to stop the motion of a chair without regard for whether the assistant's instrument holder or the like is pressed from above or below by the motion of the chair.

CONSTITUTION: An assistant's instrument holder is mounted on a holding member 13 via an arm 12. The arm 12 is secured to an arm root receiving member 20 rotatably mounted on the holding member 13 by means of a shaft 21. The arm root receiving member 20 is supported by an application point member 22 attached to the holding member 13 between a holding portion Q and an arm mounting portion P and has a detection means 23 at its end portion to detect the displacement of the arm root receiving member 20. The detection means 23 stops the motion of a dental unit in operation (the motion of a chair) when the end of the arm root receiving member 20 is displaced by a predetermined amount.