1. An endoscope system comprising: an endoscope objective optical system that acquires two optical images with different focus positions from an identical object; an image pickup device that picks up the two optical images to acquire two image signals; an image synthesis processing section that makes a comparison in contrast between the two image signals for each spatially identical pixel region, and selects a pixel region having relatively higher contrast to thereby synthesize the two image signals into one image; and a focus switchover mechanism that moves a position of a focus switchover lens provided for the endoscope objective optical system and selectively switches a focus of the endoscope objective optical system to one of two observation regions of proximity observation and remote observation, wherein the image synthesis processing section synthesizes two images in each of the respective observation regions of the proximity observation and the remote observation.