PURPOSE: To simplify processing work and to attain low cost.

CONSTITUTION: A preload mechanism 23 for turnably energizing a carriage 11 is provided in the vicinity of a sub-guide shaft 22 out of two main and sub- guide shafts 19 and 22. This preload mechanism 23 is composed of a preload shaft 24 that is parallel to the sub-guide shaft 22, a preload roller 25 capable of rolling on this preload shaft 24 and a leaf spring 26 for pressing this preload roller 25 against the preload shaft 24 in contact. Thus, a traveling roller 14 and the preload roller 25 can be fitted to the carriage 11 via a traveling roller shaft 15 projecting in the horizontal direction and a preload roller shaft 27. Consequently, a roller fitting block is unnecessitated, and processing work on constitutive parts to be fitted with the traveling roller shaft 15 and the preload roller shaft 27 can simply be performed.