1. An endoscope system comprising: an endoscope which acquires a front field-of-view image and a lateral field-of-view image of an object of observation; a detecting section which has a function capable of detecting whether a treatment instrument is used or not in the endoscope based on a notification signal which is outputted when the treatment instrument is inserted into the endoscope; and an image processing section which generates an observation image including the front field-of-view image and the lateral field-of-view image in the same screen, displays a magnified image of one field-of-view image out of the front field-of-view image and the lateral field-of-view image on a display section, and performs adjacent part display processing of displaying only a part of the one field-of-view image which is adjacent to the other field-of-view image or image compression processing of displaying a compressed image of the other field-of-view image on the display section.