1. An endoscope system, comprising: an acquisition unit that acquires a first still image and a video or a second still image before and after acquisition of the first still image based on an image signal obtained by imaging an observation target using an image sensor; an association unit that associates the first still image with the video or associates the first still image with the second still image; and a storage unit that stores the first still image and the video associated with each other or stores the first still image and the second still image associated with each other.