A FIB is irradiated onto a sample to form a lamella whose upper side has a thickness of 100 nm or less and whose lower side has a thickness greater than that of the upper side. First and second measurement regions are set on an observation image of the lamella on the upper and lower sides, respectively, where the lamella is thin enough to transmit therethrough an EB. An EB is irradiated onto the first and second measurement regions and charged particles generated therefrom are detected, and a slant angle of one degree or smaller is calculated based on the detected amount of charged particles generated from the first and second measurement regions and the distance between the two regions. The lamella is slanted with respect to the FIB and then irradiated by the FIB by the calculated slant angle to uniformize the thickness of the lamella to a value of 100 nm or smaller.