This paper proposes an approach to constituting crease patterns, the unfolded origami models, by using skeletons obtained from 2-D images such as handwriting sketches. Firstly, we describe a method for constructing a data structure which represents all the parts of an origami model and their relationships based on an extracted skeleton, and then give an algorithm for constituting a crease pattern using this data structure. To show the validity of proposed method and algorithm, we finally demonstrate how a crease pattern is generated and how a origami model is actually realized using some real illustrations taken from a origami drill book.