A temperature estimation method of estimating a temperature of a heat generating site in an oil-immersed electric appliance immersed in insulating oil, the insulating oil being silicone oil or ester oil, the temperature estimation method comprising: - a measurement step of measuring concentrations of two types of thermal decomposition products in the insulating oil; and - a calculation step of calculating a temperature of the heat generating site in the oil-immersed electric appliance based on a concentration ratio between the two types of thermal decomposition products and a relational expression between the temperature of the heat generating site and the concentration ratio prepared in advance, characterized in that - at least one of the two types of thermal decomposition products is straight-chain siloxane, alcohol containing silicon, or benzene when the silicone oil is adopted as the insulating oil, and - the two types of thermal decomposition products are fatty acids when the ester oil is adopted as the insulating oil, wherein the concentration ratio is a concentration ratio of hexamethylcyclotrisiloxane/trimethylsilanol, benzene/hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane/trimethylsilanol, benzene/octamethylcyclotetrasiloxane, octamethyltrisiloxane/hexamethyldisiloxane, or benzene/trimethylsilanol when the silicone oil is adopted as the insulating oil, wherein the concentration ratio is a concentration ratio of octamethyltrisiloxane/hexamethylcyclotrisiloxane, octamethyltrisiloxane/octamethylcyclotetrasiloxane, hexamethyldisiloxane/trimethylsilanol, or octamethyltrisiloxane /trimethylsilanol when the silicone oil is adopted as the insulating oil in a temperature region of over 500 °C, or wherein the concentration ratio is a concentration ratio of branched fatty acid/straight-chain fatty acid, straight-chain fatty acid/branched fatty acid, saturated fatty acid/unsaturated fatty acid, or unsaturated fatty acid/saturated fatty acid when the ester oil is adopted as the insulating oil. - a measurement step of measuring concentrations of two types of thermal decomposition products in the insulating oil; and - a calculation step of calculating a temperature of the heat generating site in the oil-immersed electric appliance based on a concentration ratio between the two types of thermal decomposition products and a relational expression between the temperature of the heat generating site and the concentration ratio prepared in advance, characterized in that - at least one of the two types of thermal decomposition products is straight-chain siloxane, alcohol containing silicon, or benzene when the silicone oil is adopted as the insulating oil, and - the two types of thermal decomposition products are fatty acids when the ester oil is adopted as the insulating oil, wherein the concentration ratio is a concentration ratio of hexamethylcyclotrisiloxane/trimethylsilanol, benzene/hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane/trimethylsilanol, benzene/octamethylcyclotetrasiloxane, octamethyltrisiloxane/hexamethyldisiloxane, or benzene/trimethylsilanol when the silicone oil is adopted as the insulating oil, wherein the concentration ratio is a concentration ratio of octamethyltrisiloxane/hexamethylcyclotrisiloxane, octamethyltrisiloxane/octamethylcyclotetrasiloxane, hexamethyldisiloxane/trimethylsilanol, or octamethyltrisiloxane /trimethylsilanol when the silicone oil is adopted as the insulating oil in a temperature region of over 500 °C, or wherein the concentration ratio is a concentration ratio of branched fatty acid/straight-chain fatty acid, straight-chain fatty acid/branched fatty acid, saturated fatty acid/unsaturated fatty acid, or unsaturated fatty acid/saturated fatty acid when the ester oil is adopted as the insulating oil.