1. A pattern defect detecting method for detecting a defect in main patterns formed on a photomask to be transferred to photosensitive material, comprising the step of: scanning said photomask using a mask pattern defect inspecting apparatus, one or more sub-patterns being previously formed inside a rectangular region including all of said main patterns therein, in a manner that a part of at least one pattern selected from a group composed of said main patterns and said sub-patterns is included in a prescribed scan target range of said mask pattern defect inspecting apparatus at least in said rectangular region.