1. A method of measuring pattern dimensions, comprising the steps of: obtaining an image of a region on a sample in which a measurement target pattern is formed, by irradiating and scanning the region with a focused charged particle beam and detecting secondary charged particles generated from the sample; forming an added signal waveform of the measurement target pattern by adding plural signal waveforms of the obtained image to a direction perpendicular to the scanning direction of the charged particle beam; and obtaining dimension information of the measurement target pattern from the added signal waveform of the measurement target pattern, wherein, in the step of forming an added signal waveforms, the plural signal waveforms to be added are corrected in deviation of the waveforms corresponding to edge positions caused by edge portion roughness of the measurement target pattern.