PURPOSE:To easily obtain the sample to be used to diagnose the presence or not of abnormality inside a tank by a method wherein the gas on the oil surface in the tank of an oil-immersed machine is introduced to the prescribed outside position within easy reach by hand through a gas piping, and the gas is collected there. CONSTITUTION:The temperature of insulating oil 3 is taken by a thermometer through the intermediary of a temperature sensing part 14. Then, valves 7A and 7B are opened, a gas pump 9 is driven, gas is circulated in gas space 4, and the gas is replaced with the gas of a gas collecting hole 10. The gas in the gas collecting hole 10 is collected by means of a syringe with a lock, and the collected gas is passed through a gas chromatograph. As the gas component in the gas space 4 and the component of the gas in oil are balanced, the component and the quantity of the gas in oil can be estimated by analyzing the gas, and the presence of abnormality in a transformer can also be diagnosed.