PURPOSE: To detect on abnormal condition in an oil-immersed transformer by estimating the mechanical deterioration of the insulating paper in the transformer from a total amount of CO2 and CO.

CONSTITUTION: Insulating oil 2 is fed into an oil collecting and gas extracting section 10 through an oil supply tube 8 and gas is extracted from the oil. The extracted gas 13 is then guided to a gas chamber 11a. An infrared radiation generator 11c is activated by a signal from a control circuit 11b so that infrared radiation of a predetermined wavelength is applied to an infrared sensor 11d via the extracted gas 13. Since the infrared radiation is absorbed by CO2 or CO gas, output of the sensor 11d is inversely proportional to an amount of CO2+CO gas contained in the extracted gas 13. The output of the sensor 11d is received by an arithmetic processor section 11f through an amplifier 11e and the amount of gas is calculated by the processor section 11f for estimating deterioration in mechanical strength of an insulation paper in the tested transformer. The processor section 11f compares the estimated value with a limit value of mechanical strength of the insulation paper which has been previously set in the processor section 11f. Based upon a result of the comparison, it is determined whether the transformer has any abnormal condition or not.