Upper and lower wire guides in a wire-cut electric discharge machine are moved until a wire electrode contacts with a workpiece, and after the contact, the upper and lower wire guides are moved backward to a position A which is away from the contact point at a fixed distance. Then, the upper wire guide is moved until the wire electrode contacts with the workpiece and the upper wire guide is returned to the position A. Subsequently, the lower wire guide is moved until the wire electrode contacts with the workpiece and the lower wide guide is returned to the position A. A coordinate value on the position where the wire electrode contacts with the workpiece is calculated by using position detection values which are obtained in the above respective steps, and board thickness is calculated from the coordinate value.