A mark position detection system used in a charged particle beam apparatus and including detection circuit for detecting a reflected electron generated at a mark when the mark is scanned with a charged particle beam, to obtain a mark signal, and signal processing circuit for comparing the mark signal from the detection circuit with a predetermined threshold level to find the position of the charged particle beam at a time the mark signal traverses the threshold level, thereby detecting the position of a mark edge, is disclosed in which, when the mark signal traverses the threshold level and has a peak value exceeding a predetermined value, it is determined by the signal processing circuit that the position of the charged particle beam at a time point at which the mark signal traverses the threshold level is the position of the mark.