A method for correcting a drift of an accelerating voltage includes measuring, after a position of a focus of a charged particle beam has been adjusted based on a first adjustment value and a predetermined time period has passed, a second adjustment value when the position of the focus of the charged particle beam is newly adjusted, calculating a deviation amount between the first adjustment value and the second adjustment value, calculating, using a correlation stored in a storage device, a correction value of an accelerating voltage to be applied to a beam source which emits the charged particle beam, where the correction value corresponds to the deviation amount, and correcting the accelerating voltage to be applied to the beam source, by using the correlation value.