To prevent reduction of the quality of a woven fabric caused by sagging of a selvage by automatically detecting the yarn break of a selvage warp thread carrying out a shedding operation by moving upward and downward, and stopping the loom.

A selvage warp thread sensor 16 is installed in a home position at which a pair of the selvages can be detected in a selvage device 1. The output terminal of the selvage warp thread sensor 16 is connected to an input terminal of a circuit 31 for detecting abnormality of the selvage warp thread to detect the pair of the selvage warp threads 2 moving upward and downward according to the weaving operation. The circuit 31 for detecting the abnormality of the selvage warp thread having the detected signal of the selvage warp thread sensor 16 inputted thereto compares a generated form of a detected signal from the selvage warp thread sensor 16 at normal weaving time, with a generated form of a detected signal from the selvage warp thread sensor 16 at abnormal weaving time, and outputs a signal of abnormality in the selvage warp thread when the generated forms are different.