Endoscope system for generating a composite image from optical images of different focus

There is provided an endoscope system in which a depth of field is widened, and which is capable of acquiring a high-quality image with an aberration corrected favorably. The endoscope system includes an objective optical system OBL, an optical path splitting unit 20 which splits an object image acquired by the objective optical system OBL into two optical images having a different focus, an image pickup element 22 which acquires the optical images, and an image synthesis processing section 23c which selects in a predetermined area an image with a relatively high contrast from the two optical images acquired, and generates a composite image, wherein the objective optical system OBL includes a first lens L1 having a negative refractive power which is nearest to object, and the endoscope system satisfies the following conditional expressions (1) and (2). 3 < D _ diff / im _ pitch < 100 0.005 < D _ diff / R 1 _ r < 1.0 where, D_diff denotes an air conversion length of a difference in length of optical paths of the two optical images, im_pitch denotes a pixel pitch of the image pickup element, and R1_r denotes a radius of curvature at an image-side surface of the first lens having a negative refractive power.