1. A pattern inspection method comprising: taking a plurality of images concerning the same inspection target pattern at different times by use of an imaging apparatus; detecting edges of the obtained images, respectively; matching the detected edges of different imaging times; obtaining a difference between corresponding edges to generate a difference vector after the matching; predicting an edge shape of the inspection target pattern at an arbitrary future time based on the generated difference vector and an interval between the imaging times; setting a threshold corresponding to a required specification of the inspection target pattern; and predicting the time when the inspection target pattern fails to meet the required specification from the predicted edge shape and the threshold.