1. An endoscope system, comprising: an endoscope comprising: an insertion portion configured to be inserted into a subject; and an image sensor configured to pick up an image of inside of the subject; and a processor comprising hardware, wherein the processor is configured to: acquire positional information of the image sensor; acquire insertion form information of the insertion portion inside the subject; on a three-dimensional model image that simulates a predetermined organ inside the subject, superimpose an insertion form image that is based on the insertion form information of the insertion portion inside the subject, and based on the positional information of the image sensor, also paste an image pickup image that is picked up by the image sensor, and present the three-dimensional model image; and present the image pickup image that is picked up by the image sensor and further, based on the positional information of the image sensor and specific image information among images that are pasted by the processor, present directional information indicating a direction in which the specific image information exists relative to the image pickup image.