A method of performing treatment on a desired part of a cavity includes use of an endoscope having a guide wire, a therapeutic instrument and a therapeutic instrument elevator base capable of guiding the instrument to a desired position. The guide wire is inserted into an insertion channel of the endoscope and located in a desired position in the cavity. The instrument is moved from a position in the cavity to a position closer to the operation portion of the endoscope than to the distal end of the endoscope. While a guide wire fixing means at the distal end of the endoscope fixes the guide wire, the instrument is completely pulled out from the instrument channel of the endoscope.