The instrument has an original cleaning system, which includes a gas nozzle supplying a gas jet onto the optical surface under high pressure. This pressure is sufficient to enable the gas jet to be a single cleaning means for cleaning the optical surface in all clinical instances. A safety means prevents the patient's internal organs from barotrauma by the gas jet and includes a gas jet catcher and a suction pump of ejection type. The latter is arranged in the instrument's handle and can be used also for the removal of abundant secretions or body's small particles from the patient's cavity. Besides, the gas jet and the suction pump along with a control means form the system for pressure control within the patient's cavity.