There is provided a primary beam scanning apparatus capable of producing data having a bit depth higher than the resolution of an analog-to-digital converter. The primary beam scanning apparatus is capable of controlling a scan rate of a primary beam for scanning a specimen. The scanning apparatus includes a detector for detecting signals generated in response to irradiation of the specimen with the primary beam and providing an analog output signal, an analog-to-digital converter for sampling the analog signal and converting it into a digital signal, and an arithmetic section for averaging the digital signal.