1. An image capturing device, comprising: an image capturing element that captures an image of light from a subject; a first image processing unit that processes a plurality of frame images to form a moving image file, on the basis of an image capturing signal from the image capturing element; an instructing unit that instructs to capture a still image of the subject; a second image processing unit that processes the still image on the basis of the image capturing signal from the image capturing element, wherein when the instructing unit instructs to capture the still image during capturing of the moving image, the second image processing unit processes the still image and then generates an interpolation frame image, which is the frame image until the capturing of the moving image restarts after the instructing unit instructs to capture the still image, while the first image processing unit simultaneously performs the capturing of the moving image that is restarted immediately after the still image is captured; and a moving image file generating unit that generates the moving image file, on the basis of the plurality of frame images processed by the first image processing unit and the interpolation frame image generated by the second image processing unit.