To provide a levelling string setting device for easily setting a levelling string on a horizontal bar member such as a cradling support to be installed on a ceiling slab with suspension bolts.

The levelling string setting device comprises two suspension bodies 21a, 21b with hooks 26 to be mounted on the horizontal bar member 12 in a suspended manner. One suspension body 21a has a levelling string holder 31 at the lower part, to which the levelling string 100 is connected at one end and in which a winding reel 33 is built for winding the leveling string in a free condition with predetermined tension. In a side portion of the holder 31, a leveling string access 35 is provided from which the levelling string 100 can be drawn at the other end being kept at a fixed level. The other suspension body 21b has a length corresponding to the level of the leveling string access 35 when both suspension bodies are mounted on the horizontal bar member 12 in the suspended manner, and the levelling string 100 is connected at the other end to the level of the levelling string access 35. The levelling string can be set simply by mounting both suspension bodies on the horizontal bar member 12 with the hooks 26.