1. An endoscope system configured to analyze, when an endoscope is inserted into a subject, an observed region and an unobserved region of the endoscope using information of a three-dimensional image of the subject obtained in advance, the endoscope system comprising: one or more processors comprising hardware, wherein the one or more processors are configured to: extract a predetermined luminal organ existing in the three-dimensional image, as a luminal organ model; acquire information of an observation position and a visual line direction for performing observation inside the subject by using an objective optical system; generate boundary position information indicating a boundary position of an angle-of-view region and another region of the objective optical system on the three-dimensional image of the subject based on the information of the observation position and the visual line direction that change along with temporal change observation and based on angle of view information indicating an angle of view in the observation using the objective optical system; and extract a non-angle-of-view region not in the angle of view on the three-dimensional image based on the boundary position information, wherein the processors are further configured to: be able to three-dimensionally change a display direction for displaying the luminal organ model, and when an identification image provided with identification information is generated on the luminal organ model with the display direction changed, generate the identification image in a direction such that a proportion of the non-angle-of-view region in the identification image is largest.