A focus adjustment method for a charged particle beam device having a magnetic field lens used for focus adjustment and an astigmatism corrector includes: acquiring a plurality of first images by varying an excitation current of the magnetic field lens within a focus search range, and determining a reference value of the excitation current; removing hysteresis from the magnetic field lens by setting the excitation current of the magnetic field lens outside the focus search range before and after varying the excitation current of the magnetic field lens within the focus search range; and acquiring a plurality of second images by varying the excitation current of the magnetic field lens using the reference value as a reference and varying a stigma correction value of the astigmatism corrector at each excitation current, and then determining optimum values of the excitation current and the stigma correction value.