To provide a back-filling injection device having high durability and reliability by avoiding the intrusion of a back-filling material into the sliding gap between members.

The back-filling material fed from a back-filling material feed pump 7 is injected and filled into the space section between the ground 21 and the back section of a segment 20, then the inside of a back-filling material feed pipeline 13 is cleaned with the detergent fed from a detergent feed pump 8 in this back-filling injection device. A selector valve device 1 switching the feed or stop of the back-filling material and the detergent is arranged on the outer wall 6 of a machine body, and the selector valve device 1 is formed into a rotary sliding type.