1. A fake finger determination device, comprising: acquisition unit for acquiring line width information related to a line width of a ridge or a line width of a valley line of a fingerprint based on a fingerprint image; and determination unit for determining whether or not the finger corresponding to the fingerprint image is a real finger or a fake finger based on a relation of the line width of the ridge and the line width of the valley line, wherein the acquisition unit further comprises: splitting unit for splitting the fingerprint image into small regions; deciding unit for deciding, for each of the split small regions, a line direction of the ridge or the valley line in the region; gradient derivation unit for obtaining a maximum value and a minimum value of a gradient of changes in a pixel value by scanning each of the small regions in a direction that is perpendicular to the decided line direction in the region; and line width calculation unit for obtaining the line width information by setting, as a line width starting point, a point where the gradient of changes in the pixel value obtained by scanning each of the small regions in the direction perpendicular to the line direction takes the maximum value, setting a point where the gradient of changes takes the minimum value as a line width end point, and counting the number of pixels from the line width starting point to the line width end point.