A small observation optical system having a high resolution, an observation imaging device that includes the observation optical system, an observation imaging system, an image forming lens system, and a method of adjusting the observation optical system. The observation optical system includes an objective lens system G1 and an image forming lens system G2, sequentially from the observation object side, and the image forming lens system G2 forms an observation object image forms through the objective lens system G1, on an image plane IP of an image sensor, and satisfies predetermined condition.