To provide a yarn feed switching detector capable of identifying the causes of yarn break in a fiber processing machine such as a drawing false twister which is essential for stabilization of process, improving the efficiency of switching operation between yarn feed packages, and detecting switching between the yarn feed packages in real time.

This yarn feed switching detector detects yarn feed switching in the fiber processing machine, formed so that, with at least two yarn feed package disposed for each of spindles, a yarn line may be continuously fed to a yarn feeding point by switching from one yarn package to the other with the yarn end of one package tied to the yarn beginning of the other, is provided with a yarn guiding means guiding a yarn line from each of the yarn feed packages rotatably-supported in each of the directions of the yarn feed packages around a fulcrum formed near the yarn feed point to the yarn feeding point, and a guide detector detecting switching of the yarn guiding means in each of the directions of the yarn feed packages.