A charged particle beam apparatus with improved depth of focus and maintained/improved resolution has a charged particle source, an off-axis illumination aperture, a lens, a computer, and a memory unit. The apparatus acquires an image by detecting a signal generated by irradiating a sample with a charged particle beam caused from the charged particle source via the off-axis illumination aperture. The computer has a beam-computing-process unit to estimate a beam profile of the charged particle beam and an image-sharpening-process unit to sharpen the image using the estimated beam profile.