1. A method for inspecting a defect in a photomask which is produced by using graphic data, that match mask data or are produced by subjecting mask data to correction of a process conversion difference relating to at least a line portion width, the method comprising: producing inspection data by correcting a pattern of mask data so as to substantially match a planar shape of a pattern of a photomask to be produced by using the graphic data; comparing a pattern of a produced photomask with a pattern of the inspection data; extracting portions where planar shapes of the pattern of the inspection data and the pattern of the produced photomask do not match; and distinguishing a defect from the portions where the planer shapes do not match.