1. An endoscope system comprising: an insertion section configured to be inserted into a subject; an objective optical window provided on a distal end side of the insertion section and configured to receive light from the subject; an image pickup sensor configured to pick up an image inside the subject from the light made incident from the objective optical window; a position-information acquiring sensor configured to acquire position information of the objective optical window; and a central processing unit configured to: control a memory to record an intra-subject image picked up by the image pickup sensor and the position information acquired by the position-information acquiring sensor in association with each other; associate the position information recorded in the memory with a model image of a predetermined organ in the subject; generate, as a pasted image, an image obtained by pasting the intra-subject image onto the model image of the predetermined organ; determine whether a predetermined trigger signal concerning the image pickup in the image pickup sensor is generated or a predetermined feature value of the intra-subject image picked up by the image pickup sensor satisfies a predetermined condition; and apply processing for pasting the intra-subject image, concerning which the predetermined trigger signal is determined as being generated or the predetermined feature value of which is determined as satisfying the predetermined condition, onto a forefront on the pasted image relative to other intra-subject images and apply image processing to the intra-subject image pasted onto the forefront to be distinguishable from the other intra-subject images.