Disclosed herein is a presumably defective portion decision apparatus, including: an arithmetic operation section configured to divide a level difference included in level difference data which indicate a level difference distribution on the surface of a semiconductor device into two or more unit level differences in the depthwise direction of the level difference and determine, for each of the unit level differences obtained by the division, a relationship between the height of a contour line at a level difference position of an upper face and an area of an opening surrounded by the contour line to decide presence or absence of a presumably defective portion.