1. A stereoscopic image printing device, comprising: an image input unit configured to input a multi-viewpoint image with two or more viewpoints; an information acquisition unit configured to acquire information on distribution of parallax of the inputted multi-viewpoint image; a number-of-viewpoint determination unit configured to determine, based on the information on the distribution of parallax acquired by the information acquisition unit, a number of viewpoints of a stereoscopic image which is printed on a lenticular lens sheet; a comparison unit configured to compare a number of viewpoints of the multi-viewpoint image inputted via the image input unit and the number of viewpoints determined by the number-of-viewpoint determination unit; a viewpoint image generation unit configured to generate, if the number of viewpoints of the multi-viewpoint image inputted via the image input unit is smaller than the number of viewpoints of the stereoscopic image which is printed on the lenticular lens sheet, a shortfall viewpoint image based on the inputted multi-viewpoint image; and a printing unit configured to print the stereoscopic image including the multi-viewpoint image inputted via the image input unit and the viewpoint image generated by the viewpoint image generation unit, on the lenticular lens sheet.