To inhibit a chair from escaping (moving) backward when a dental assistant sits in the chair even in a slightly wrong attitude.

The chair 20 for the dental assistant includes casters 24 (241-245). When a load is applied to these casters 24 (241-245) or front casters 241, 242, and 243, the casters 24 are locked. When its seat 21 is of a rotary type, all the casters 241-245 are each formed with a locking mechanism. When the seat 21 is of a non-rotary type, the front three casters 241-243 are each formed with the casters. Thus, even when the assistant sits in the chair shallowly from an oblique front side, the chair 20 does not move backward, and the assistant can sit on the chair safely free from anxiety.