A method for accurately diagnosing an internal fault of an oil-immersed transformer by analyzing content ratios of dissolved gases generated when the internal fault occurs in the oil-immersed transformer is provided. The method diagnoses the internal fault by analyzing the dissolved gases contained in the insulating oil of the oil-immersed transformer for internal fault diagnosis, wherein the method comprises: a first step of extracting H2, CH4, C2H4, and C2H2 from the dissolved gases; a second step of calculating a content ratio (%) of each dissolved gas from the total content of the four dissolved gases selected from the extracted five dissolved gases; and a third step of determining the internal fault of the oil-immersed transformer for diagnosis corresponding to an internal fault region according to content ratio (%) values of the calculated four dissolved gases and predetermined content ratios (%) of the four dissolved gases.