1. An endoscope system comprising: an endoscope having an insertion portion to be inserted into a subject; an objective optical system provided in the insertion portion and configured to form an optical image of an inside of the subject; and one or more processors comprising hardware, wherein the one or more processors are configured to: acquire position information about the objective optical system; position the position information about the objective optical system relative to three-dimensional image information related to the subject; generate angle-of-view display information showing angle-of-view information about a picked-up image picked up by an image pickup system including the objective optical system; and automatically create an overlapped image obtained by overlapping the angle-of-view display information at a position on the three-dimensional image information corresponding to the position information about the objective optical system, if a duration while the objective optical system is judged to be present in a predetermined area is equal to or above a first threshold based on the position information, after the duration until a duration while the objective optical system is judged to be absent in the predetermined area is equal to or above a second threshold.