1. An image classification device comprising: a processor; and a non-transitory memory having stored thereon executable instructions, which when executed, cause the processor to function as: an image specification unit operable to specify, among a plurality of images included in an image group which have been sequentially photographed with respect to one event, one or more images in which a human face having a predetermined feature appears; a reference value calculation unit operable to calculate a reference value, by dividing a sum of a number of the plurality of images and a value of 2 by a sum of a number of the specified images and a value of 1; a difference value calculation unit operable, with respect to each of consecutive-image pairs that are each composed of two images consecutive in order of photographing time included in the images specified by the image specification unit among the plurality of images included in the image group, to calculate a difference value indicating a difference between numbers assigned to the respective two consecutive images arranged in order of photographing time; an evaluation value calculation unit operable to calculate an evaluation value based on a degree of dispersion of the specified images, the evaluation value indicating an importance degree in the image group with respect to a person specified by the human face having the predetermined feature; and an image group classification unit operable to classify the event relating to the image group based on the evaluation value calculated by the evaluation value calculation unit, wherein the evaluation value calculation unit compares each of the difference values calculated by the difference value calculation unit with the reference value calculated by the reference value calculation unit, and calculates the degree of dispersion value based on a number of consecutive-image pairs each having a difference value that is greater than the reference value.