PURPOSE: To enable to easily monitor the running thread length, thread speed and thread cut, etc. by running thread in a bundle of beam from plural slits having filter element through a cylindrical transparent guide.

CONSTITUTION: A differential type space filter element 2, which is provided together with an amplifier 4 and a transparent guide 6 in a closed container 5, is constituted of the combination of a slit row group A consisting of slit rows a which are united in a body respectively and arranged continuously in parallel and a slit row group B consisting of slit rows b. These slit rows a, b are formed of photosensitive conductive material such as solar battery, etc. respectively and arranged alternately and the differential type filter element 2 detects the difference between signals composed by both slit row groups A, B. And thread 3 is made to run inside a cylindrical transparent guide 6 and the beam from a laser emitting element 1 is irradiated onto the differential type space filter 2 and its signal is input into an outside counter through an amplifier 4. In this way, thread cut, skip stitch and running speed are checked by monitoring the existence of a pulse arrow.