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; a position information acquiring portion configured to acquire position information about the objective optical system; a positioning portion configured to position the position information acquired by the position information acquiring portion relative to three-dimensional image information related to the subject; an angle-of-view display information generating portion configured to 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; an instruction portion configured to be operable by a user and output an instruction signal giving an instruction to overlap the angle-of-view display information generated by the angle-of-view display information generating portion on the three-dimensional image information; and an image processing portion configured to 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 acquired at a time point of inputting the instruction signal by the instruction portion.