To move a main body section of a treatment chair in the back and forth direction associated with a tilting action of a dental chair which can slant (tilt) so that a position of the mouth of a patient in the back and forth direction does not change regardless of a situation of an inclination.

The dental chair is comprised of the main body section 10 of the treatment chair having a contour seat 13 and a backrest 12 and a base 20 to mount and support the above main body section 10 to be moved in the anteroposterior direction, wherein the main body section 10 has a rotary shaft Q at a joint section of the contour seat and the backrest and the main body section 10 is slanted by the rotation of the rotary shaft Q. The rotary shaft Q has a gear 2 and the base 20 has a rack 3 to engage with the gear 2 so that when the rotary shaft Q is rotated and the main body section 10 is slanted (when the situation (A) of the figure is changed to the situation (B) of the figure), the main body section 10 is moved to the front or the back on the pedestal by the engagement of the gear 2 and a rack tooth 3a and the position of the mouth of the patient K1 and K2 does not change in the anteroposterior direction regardless of the slanting of the main body section 10.