Device for measuring size of hole behind tunnel lining

A tunnel lining back cavity size measuring device comprises a box body, a transmission mechanism, an induction sheet and a spring steel tape. The transmission mechanism is arranged in the box body, and two ends of the transmission mechanism are erected on two side walls of the box body; the spring steel measuring tape is wound on the transmission mechanism in the box body, one end of the spring steel measuring tape extends out of a gap in the top of the box body, and a sensing piece is arranged at the top end of the spring steel measuring tape; a counting meter is arranged on the box body; one end of the transmission mechanism extends out of the box body and is connected with a hand rocker, a circular graduated scale is arranged on the side wall of the box body close to the hand rocker, and a pointer is arranged on the hand rocker; the pointer is electrically connected with the counting meter; when the hand rocker drives the transmission mechanism to rotate, the top end part of the spring steel tape drives th