The manometer, attached to the instrument for artificial pneumothorax, is used for measuring the breathing pressure in a thorax cavity. On that occasion, the pneumothorax needle forms a neck of air flow and the manometer has its own property. So it is inevitable that the indication of manometer surface is attended with consider able error and time lag. Author made some experiments on the relation between pressure variation and manometric indication by means of photographic self-recorder, and made the comparative study of the experimental results and the theoretical ones.