An image capturing device includes: an illuminator configured to generate illumination light having wavelength bands and an illumination characteristic; an imager configured to generate an image signal by capturing light from a subject; and a processor configured to: control the illumination characteristic; set a piece of statistical data used for estimating a spectral characteristic of the subject included in the image signal; estimate the spectral characteristic of the subject included in the image signal based on the set piece of statistical data; define an illumination characteristic of illumination light in accordance with the estimated spectral characteristic of the subject; and determine whether or not the set piece of statistical data is an adequate piece of statistical data based on whether or not an image signal generated by the imager under the illumination light having the defined illumination characteristic satisfies a criterion for achromatic color.