According to a body cavity interior observing apparatus of the present invention, a non-tracked region to be displayed can be switched between a first non-tracked region and a second non-tracked region which is larger than the first non-tracked region, and in a search for an affected area in which it is preferable to track a treating instrument in order to consistently align the position of treating instrument to the central part of the screen, the first non-tracked region having a smaller non-tracked region is selected, while in a treatment of an affected area in which a treating instrument is moved in a wide range of area, the second non-tracked region having a larger non-tracked region is selected, thereby an appropriate monitor screen can be provided in searching for an affected area and in treating an affected area.