A relationship between a weight of a reel (7) supported by a column of a wire electric discharge machine when a wire electrode (3) is wound around the reel, and an amount of slanting of the wire electrode (3) when the wire electrode extended between an upper wire guide (5) and a lower wire guide (6) becomes slanted as a result of the column being deformed by the weight of the reel (7) is stored. The weight of the reel is determined during electric discharge machining, and the amount of slanting of the extended wire electrode is determined from the weight of the reel. A compensation amount for the slanting of the wire electrode is determined from the amount of slanting, and the slanting of the wire electrode is compensated by the compensation amount.
