1. A computer-implemented method of determining a layout of a plurality of product plates of various sizes on multiple material plates, the method comprising the steps of: generating a plurality of layout patterns each including at least one of the plurality of product plates to be cut from a material plate, the generating comprising: selecting, by the computer, at least one candidate product plate from the plurality of product plates as a candidate to be included in a layout pattern; determining, by the computer, whether or not to generate the layout pattern including the at least one candidate product plate selected in the selecting step based on a product plate evaluation value indicative of a number of times the each product plate has been included in previously-generated layout patterns; provided that a determination is made to generate the layout pattern in the determining step, generating, by the computer, the layout pattern including the at least one candidate product plate selected in the selecting step; and updating, by the computer, the product plate evaluation value of each of the at least one product plate for use in a next layout pattern generation, wherein in the product plate selecting step, selecting by the computer at least one candidate product plate from the plurality of product plates of various sizes so that a combined size of the at least one candidate product plate is within a predetermined set size, and in the determining step, determining by the computer whether or not to generate the layout pattern based on a layout pattern evaluation value which depends on a filling ratio indicating a ratio of the combined size of the combined at least one candidate product plate selected in the product plate selecting step, to the predetermined set size, and further based on a mean value of the product plate evaluation value of the at least one candidate product plate selected in the product plate selecting step.