In oil-filled equipment such as transformers, local overheating or discharges will precede a final shutdown. Along with such minor internal problems, decomposition of the insulating materials into gases, which are dissolved into the insulating oil, occurs. For preventive maintenance of transformers, dissolved gas analysis is currently made using a gas chromatograph, which takes time to analyze. Since hydrogen gas is always one of the decomposed gases, a portable oil-dissolved hydrogen gas detector was developed for quick field screening of oil-filled equipment for detailed inspection. The operation and performance of this detector are described, and comparison with existing methods is made.ETX