In a rack extension jig device 9, a protracted direction E of tooth projections 12-14 and an elevational direction H of racks 7, 8 are placed in a coplanar position G. This makes it possible to positively engage the tooth projections 12-14 with the respective rack teeth 7, 8 and half tooth surfaces 7c, 8c so as to set a predetermined pitch interval T at a contact portion between the neighboring racks 7, 8 by only pushing the tooth projections 12, 13, 14 against roots 7d, 8d of the respective rack teeth 7, 8 in perpendicular to the roots 7d, 8d, thereby making a rack-adding work easy, quick and user-friendly.