The present invention is an endoscope apparatus including an endoscope (1) comprising an observation optical system (25) in which the angle of observable field of view for observing a therapeutic tool can be changed and an operating mechanism for changing the observable field of view and an insertion guide tube (2a,2b) for guiding the therapeutic tool into a body cavity, wherein the insertion guide tube (2a,2b) is detachably positioned and supported in a predetermined position relative to the endoscope (1). With this apparatus, the therapeutic tool can be observed in an optimal observing state irrespective of whether the area to be observed is located near or far from a distal end of the endoscope (1).
