1. An endoscope system comprising: a light source for emitting an illumination light to illuminate an living organism; an insertion part to be inserted into the living organism for guiding the illumination light emitted from the light source and a light returned from the living organism; a deflection optical system being provided at a tip of the insertion part for outwardly directing the illumination light guided through the insertion part in a direction of a radius of the insertion part and for introducing a light inwardly returned from the living organism in the direction of the radius of the insertion part into the insertion part; an image capturing part for capturing the returned light guided through the insertion part; an insertion speed detection part for detecting an insertion speed of the insertion part; a frame rate adjustment part for adjusting a frame rate for the image capturing of the image capturing part based on the insertion speed detected by the insertion speed detection part; an image forming part for arranging strip-shaped images acquired by the image capturing part at a pitch based on the insertion speed to form an image of the inside of the living organism; and a display part for displaying the image formed by the image forming part.