PURPOSE: To monitor a gas dissolved in a liquid and record its amount at all times, by a method wherein a chamber used to collect a gas passing through a high molecular film which can pass the gas dissolved in the liquid and remaining therein but does not pass liquid particles, and a hollow body made by closing the ends of the high molecular film is inserted into the liquid.

CONSTITUTION: For the measurement of a gas dissolved in a liquid, for instance, in the case of a liquid enclosed in equipment such as a transformer, a high molecular film 4 made of polyolefin, polysulfon, etc. which do not pass liquid particles but the gas, is attached to bolts 3, 3' by supporting the film with pipes 5, 6 with a number of holes in the parts to be soaked in the liquid 1, allowing the bolts to penetrate through the wall of a liquid container. The inside pipe 6 is connected to a part where the pressure is to be detected, such as a semiconductor type pressure sensor, whereas the pipe 6 and a gas collecting chamber 8 are formed. When the amount of the gas within the liquid 1 is measured, a cock 10 is opened to release the gas in the chamber 8 from the pipe 9. Then the pressure in the sensor is balanced and reduced to zero, and the cock 10 is closed to collect the gas allowed to pass the film 4. By obtaining the density of the gas in the liquid and the pressure of the gas having penetrated beforehand, the gas can be monitored and recorded at all times.