Provided is an imaging optical system for realizing an endoscope that is sufficiently small, sufficiently wide-angle, and sufficiently high-resolution. The imaging optical system is provided with a first lens having a negative refractive power, a second lens having a positive refractive power, an aperture stop, and a third lens having a positive refractive power, disposed from the object side to the image side. With d being the distance between the image-side principal point of the first lens and the object-side principal point of the second lens, d = -d being the signed distance between the image-side principal point of the first lens and the object-side principal point of the second lens when the image-side principal point of the first lens is further towards the image side than the object-side principal point of the second lens, and f12 being the composite focal length of the first lens and the second lens, the relationships d <； 0 and 0.005 <； d / f12 <； 16 are established.
