An object of the present invention is to measure a landing angle even in a multi electron beam lithography system in which current amount of each beam is small. Another object thereof is to measure an absolute value of the landing angle and a relative landing angle with the high SN ratio. In a transmission detector including two diaphragm plates (first and second diaphragms) and a detector, a detection angle determined by a distance between the first and second diaphragms and an aperture diameter of the second diaphragm is made equal to or smaller than the divergence angle of the electron beam to be measured, and the landing angle is determined based on the relation between a center of the fine hole of the first diaphragm and the center of the aperture of the second diaphragm at which the amount of detected current is maximum.