1. A method for determining an array sequence of a plurality of processing operations for processing a plurality of steel plates, 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; and the method comprises the steps of: 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 within the cluster; arranging the plurality of clusters in a sequence maximizing the efficiency of processing of steel plates by regarding the first sequence constraint as a constraint between the plurality of clusters; judging whether the second sequence constraint is satisfied between a last processing operation in the first cluster and a first processing operation in a second cluster arranged next to the first cluster; and searching for another processing operation satisfying the second sequence constraint respectively with the last processing operation of the first cluster and the first processing operation of the second cluster, on condition that the second sequence constraint is not satisfied, and then arranging said another processing operation next to the first cluster and before the second cluster.