PURPOSE: To add the power upwards by using a compression ring for a head rest or for a back seat in the chair for dental treatment and to easily adjust the heat rest or the back seat at the upper position.

CONSTITUTION: The back rest is provided with a slide guide moving up and down a head rest 4 and a slide guide moving up and down a back seat 5. To the lower end of the sliding body for head rest and the sliding body for back seat, compression rings 10 and 20 are attached. Spring boards 101 and 201 pulled from the compression rings are fixed to the back seat of edge parts 102 and 202. The power is applied to the heat rest 4 and the back seat 5 and the upward position adjustment is made easy.