PROBLEM TO BE SOLVED: To provide an observation optical system having high resolution and being small, an observation imaging apparatus equipped with the observation optical system, an observation imaging system, an image-formation lens system and an adjustment method of the observation optical system.SOLUTION: An observation optical system includes, sequentially from an observation subject side, an objective lens system G1 and an image-formation lens system G2. The observation optical system is configured to cause the image-formation lens system G2 to form, on an image surface IP of an image pick-up device, an image of an observation subject taken by the objective lens system G1. In the objective lens system, an effective diameter at a closest face to an object side is smaller than an image height at a maximum viewing angle of the observation optical system at the image surface. The image-formation lens system includes at least one relay lens system configured to form the observation subject image on an intermediate image-formation face between an image-formation face and the image surface of the objective lens system. The image-formation lens system satisfies a predetermined condition.SELECTED DRAWING: Figure 1
