1. An imaging device which comprises a plurality of imaging units each having an imaging optical system and an imaging element on which a subject image is formed by the imaging optical system, so that the imaging device is capable of obtaining respective viewpoint images from the plurality of imaging units, the imaging device comprising: an image-taking magnification setting unit which varies a focal length of a first imaging unit in which the focal length of the imaging optical system is variable, in the plurality of imaging units, and thereby sets a desired image-taking magnification of the first imaging unit; a trimming area decision unit which decides a size of a trimming area which is a partial area capable of including a subject of an image-taking angle of view of the first imaging unit at a desired position, based on the image-taking magnification set by the image-taking magnification setting unit, within an image-taking angle of view of a second imaging unit having the image-taking angle of view larger than the image-taking angle of view of the first imaging unit in the plurality of imaging units; a display unit which displays a trimming range representing the trimming area having the size decided by the trimming area decision unit, and a first imaging range which is a graphic indicating the subject of the image-taking angle of view of the first imaging unit, so as to be superimposed on a second image which is a wide-angle viewpoint image obtained by the second imaging unit; a trimming position setting unit which arbitrarily sets a position of the trimming range within the second image displayed on the display unit; and an image output unit which, in response to a predetermined instruction being inputted, outputs a first image obtained by the first imaging unit in which the desired image-taking magnification is set by the image-taking magnification setting unit, and also, clips a trimming image delimited by the trimming range having the size decided by the trimming area decision unit and the position set by the trimming position setting unit, from the second image, and outputs the trimming image.