1. An imaging device comprising: an imaging unit configured to image light from an object passed through an imaging optical system including a focus lens to acquire a taken image; an optical finder with which an optical image of the object can be observed; a setting unit configured to set a manual focus mode for moving the focus lens according to an operation of an operation unit; an image generating unit configured to generate a contrast image, which is an image in which an edge of the taken image is enhanced or extracted; a displaying unit configured to display the contrast image when the manual focus mode is set; and an image superimposing unit configured to superimpose the image displayed by the displaying unit on the optical image of the object in the optical finder.