1. A nail printing device comprising: a printing head which implements printing for a nail of a finger; a fingernail image obtaining unit which picks up an image of a region containing the nail, and obtains a nail region image from the picked-up image; a nail tip region extraction unit which extracts a nail tip region of the nail, the nail tip region not overlapping a nail bed of the finger, from the nail region image; a shape adjustment-required region extraction unit which (i) positionally aligns each of a plurality of model nail images with the nail region image, (ii) extracts, as a shape adjustment-required region, a region which is located outward of an outline of the model nail image in the nail tip region and which needs to be removed, for each of the plurality of model nail images, and (iii) calculates an area of each of the extracted shape adjustment-required regions; and a nail shape designation unit which designates first, as a specific model nail image, a model nail image from among the plurality of model nail images for which the calculated area of the extracted shape adjustment-required region is the smallest; wherein each model nail image serves as a model for adjusting a shape of the nail tip region, and the plurality of model nail images are registered in advance and are different from one another in at least one of size and shape.