Abstract: A method for improving dimensional accuracy of a casting pattern. A division block used to form the casting pattern is fixed on a block fixing surface of an NC machine and then the front surface is processed, leaving a stiffness holding portion at a bottom-most portion of the division block. The division block is then turned over and the rear surface is processed, removing the stiffness holding portion. The stiffness holding portion has sufficient thickness to secure stiffness of portions surrounding a through portion, enabling withstanding of a cutting force applied to the rear surface when the division block is turned over and the rear surface processed. Thus, a deviation in position of the tool during processing of the through hole on the rear surface from that of processing of the through hole on the front surface is suppressed.