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; an imaging optical system capable of changing an imaging magnification of the observation target; an imaging magnification change operation unit for inputting an imaging magnification change instruction to change the imaging magnification to the imaging optical system; and a control unit that performs control to acquire the first still image and start acquisition of the video when the imaging magnification change instruction is detected, and to end the acquisition of the video when an operation end of the imaging magnification change operation unit is detected.