The yarn detecting system is developed using the air pressure variation caused by a yarn running through a pipe. The optimum detecting conditions for several kinds of yarns are examined. The processing technique is also investigated to judge a yarn passing through a pipe at higher degree of accuracy. The system is composed of a yarn detecting block, a pressure sensor and a personal computer. It can stably detect even 20 D spandex yarn that was impossible to be detected. Results obtained are as follows : (1) Pressure variation increases with the area which a yarn occupies in a pipe. For example, larger pressure variations are observed when a yarn has larger diameter, more filaments separate and a yarn has more fuzz. Spandex yarn which has large flexibility shows much larger pressure variation because it flows into a branch pipe. It also shows arger pressure oscillation caused by its extension and/or compression. (2) The supplied pressure to an ejector has little effect on the pressure variation except for a spandex yarn. The pressure variations are also emphasized when a branch pipe is tilted in the yarn entrance direction. (3) Pressure variations must be distinguished from the oscillations of the air flow in a pipe, because the pressure variation caused by a yarn passing through a pipe is very small. In the yarn detecting system, low-pass filter circuits and average processing with a personal computer are useful for increasing the accuracy to judge a yarn passing through a pipe.