To save time and labor to peel a nail sheet while keeping outer appearance by supplementation with a supplementary sheet without removing the nail sheet even when a nail grows after the adhesion of the nail sheet, and to reduce a cost for preparing the nail sheet.

A CPU (central processing unit) 101 prepares nail sheet picture data for printing the nail sheet on the basis of designated design data and nail shape data, and holds the preparation history of the prepared nail sheet picture data. The length of the nail which grows after the preparation of the previous nail sheet picture data is calculated on the basis of the growth rate of a prescribed nail and the preparation history. Supplementary picture data for printing the supplementary sheet to supplement the previously prepared nail sheet by adhesion onto the nail growth part is prepared on the basis of the design data used for preparing the previous nail sheet picture data specified by the preparation history and the length of the grown nail.