PURPOSE: To obtain a size sensor, having a gap for sensing the size at the end of two plate units, locating yarn thrusting into the gap at a specific turning angle at the outer peripheral edge in an approximate rectangular shape of the gap and improved in reduction of yarn trouble and stability to indication of the finest size.
CONSTITUTION: In a gauge type size sensor having a gap 2 for sensing the size at the ends of two plate units 1, the shape of the outer peripheral edge satisfying the presence of yarn thrusting into the gap 2 at a position within an operation range of upward 5° and downward 25° turning angle of the sensor at the outer peripheral edge or the lower part of the outer peripheral edge is such that the center is provided between the center of the approximate rectangular type sensor 5 and the gap 2 and a radius shorter than the longitudinal length of the gap part and longer than 1/2 or approximating thereto. Thereby, yarn trouble is reduced by the aforementioned size sensor to improve stability to indication of the finest size and the size of the yarn is sensed.