1. An image capturing device which captures an imaging target while varying a focus position and outputs at least one output image as a capturing result, the image capturing device comprising: a pre-capturing unit configured to capture the imaging target at each of different focus positions in a predetermined focus range and output, as a capturing result, a plurality of pre-captured images lower in resolution than the output image; a sharpness level computing unit configured to compute a sharpness level of each of the pre-captured images captured by the pre-capturing unit; a focus varying range determining unit configured to determine a focus position varying range within the predetermined focus range based on sharpness levels computed by the sharpness level computing unit, such that a signal-to-noise ratio of the output image is greater than or equal to a predetermined threshold; and an image capturing unit configured to capture the imaging target while varying the focus position according to the focus position varying range determined by the focus varying range determining unit, and output the at least one output image as a capturing result.