To obtain a low strength injected material to obtain enough strength by adding curved fibers into mortar or cement milk having low strength.

A compounded material comprising a solidifying material essentially comprising mortar and cement milk is used. For example, the material is prepared by compounding a liquid (A) comprising 300kg slag hardening material, 60kg of an assistant, 3l of a stabilizer, 776l of water, and 3kg of PP fiber (PP6), and a liquid (B) comprising 100l of a special water glass to obtain the total of liquids (A)+(B)=1000l. Helical fibers, ringlike fibers or fibers having at least a curved part are added to the compounded material to obtain a low strength injection material. The fiber is added by preliminarily mixing in the solidifying material such as mortar or by adding to the kneading water. Thereby, adhesion strength between the compounded material and the fibers can be improved and the obtd. injected material shows enough strength.