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 and for forming a strip-shaped image corresponding to the captured returned light; 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; a control part for correcting an exposure time of the image capturing part to be reduced and a sensitivity of the image capturing part to be increased in a case where the detected insertion speed is increased, and correcting the exposure time to be increased and the sensitivity to be decreased in a case where the detected insertion speed is decreased, the sensitivity of the image capturing of the image capturing part being adjusted corresponding to the exposure time of the image capturing part so that an intensity of the strip-shaped image formed by the image capturing part is prevented from being varied; an image forming part for arranging the strip-shaped images formed by the image capturing part at a pitch based on the insertion speed to form a two-dimensional image of the inside of the living organism; and a display part for displaying the two-dimensional image formed by the image forming part.