1. A charged particle beam device having autofocusing means that obtains a plurality of images with varying focal positions by shifting a focal position in increments of a predetermined step size and estimates a focusing position based on the plurality of images, the charged particle beam device comprising: imaging means that obtains images at a first magnification and a second magnification greater than the first magnification; focusing position analysis processing means that calculates an image feature based on a region selected from the image of the first magnification; step size determination means that sets a step size using the calculated image feature; and autofocus execution means that executes autofocusing at the second magnification using the step size that has been set.