To soften fear feeling or anxiety feeling to be received from a treatment instrument by a patient by obstructing a dental treatment instrument from a patient's visual field, to simultaneously simplify appearances of both units and to further protect the treatment instrument against a disturbance of dust, or the like.

Numeral 10 in the figure is a housing for housing the instrument, numeral 11 is an instrument holder capable of mounting the instrument and drawing the instrument forward in the horizontal direction when used, and numeral 12 is a swinging door. The door 12 has a handle 12a at its upper end, a hinge 12b at its intermediate part and a hinge 12c at its lower end. The door can be set to a closed position shown in the Fig. (A) or to n opened position shown in the Fig. (C) by lifting the door 12 or depressing the door 12. Normally, the door is set to the closed state shown in the Fig. (A) so that the instrument is not seen by the patient and simultaneously not applied by the dust, or the like.