1. A pattern data forming apparatus comprising: a data input interface that receives design data including pattern information of a specific layer of a semiconductor device and an image obtained by a scanning electron microscope; and a data calculator that forms contour data by narrowing a pattern edge included in the image, detects a correspondence between the contour data of the specific layer and the design data of the specific layer, and generates vector data of the contour data of the specific layer based on the detection of correspondence between the contour data and the design data.