In accordance with an embodiment, a signal processing method includes scanning a pattern on a substrate with a charged particle beam, detecting secondary charged particles emitted from the substrate by using a detector, outputting a signal, and filtering the signal. The detector is separated or divided into a plurality of regions, and the secondary charged particles are detected separately in each region of the detector. Intensity of the filtering is defined in dependence on a function f(θ) of an angle θ between a reference axis and a direction along which the secondary charged particles enter a detector plane. The reference axis is an arbitrary direction in a plane parallel to a surface of the substrate.