One or more inflatable balloons are provided on the outer surface of the tube of an endoscope which encloses the fiber optical light transmitting bundle. When only one balloon is provided, the balloon is provided on one side of the tube near the end thereof to enlarge the space within a body cavity in one direction and the flexible part of the tube may be bent in said direction. When moe than one balloon is provided, one of the balloons is selectively inflated to enlarge the space within the body cavity in the desired direction. In a preferred embodiment of the invention, the balloons are located at equal intervals around the tube. In another embodiment of the invention, an outer sleeve is provided around the tube with balloons on the outer face thereof and is made slidable with respect to the tube. The outer sleeve and the tube are inserted into the body cavity alternately by alternately inflating the balloons on the outer sleeve and those on the tube to facilitate the insertion thereof into the body cavity.