1. A system for determining an array sequence of a plurality of processing operations, wherein a first sequence constraint based on a first attribute of each processing operation and a second sequence constraint based on a second attribute of each processing operation are defined between each of the processing operations and each of the other processing operations, the system comprising: a classifying section classifying the plurality of processing operations into a plurality of clusters according to attribute values of the first attribute, and arranging the processing operations included in each of the clusters in a sequence satisfying the second sequence constraint; a rough-scheduling section regarding the first sequence constraint as a sequence constraint between the plurality of clusters, and arranging the plurality of clusters in a sequence maximizing the efficiency of steel plate processing; a judging section judging whether the second sequence constraint is satisfied between a last processing operation in a first cluster and a first processing operation in a second cluster arranged next to the first cluster; and a detail-scheduling section which, on condition that the second sequence constraint is not satisfied, searches for another processing operation satisfying the second sequence constraint with each of the last processing operation in the first cluster and with the first processing operation in the second cluster, and which arranges said another processing operation next to the first cluster and before the second cluster.