A temperature estimation apparatus comprising: a first temperature sensor configured to measure a first temperature (T1(t)) of a first part of a target that has a heat source; a second temperature sensor configured to measure a second temperature (T2(t)) of a second part, the second part being farther from the heat source than the first part; and a processor, wherein the processor obtains temperature-for-estimation data (dT1(t), dT2(t)), the temperature-for estimation data (dT1(t), dT2(t)) being time change quantities regarding the first temperature (T1(t)) and the second temperature (T2(t)), a thermal model includes the heat source, the first part, and the second part, in the thermal model, a first thermal resistance (Rb) represents a relation of temperatures (Tb(t), T1(t)) and heat flow (lb(t)) between the heat source and the first part, and a second thermal resistance (Rs) represents a relation of temperatures (T1(t), T2(t)) and heat flow (Is(t)) between the first part and the second part, and based on the measured first temperature (T1(t)) and the second temperature (T2(t)), the processor estimates the temperature of the heat source (Tb(t)) as a deep-body temperature of the target; characterized in that the processor extracts temperature-for-estimation data (dT1(t), dT2(t)) that are measured by the first temperature sensor and the second temperature sensor, respectively, and meet a predetermined certain condition, the certain condition corresponding to a certain state of change in a temperature of the heat source (Tb(t)), based on the obtained temperature-for-estimation data (dT1(t), dT2(t)); the processor obtains a thermal resistance parameter that represents a relation between the first thermal resistance (Rb) and the second thermal resistance (Rs) in the thermal model based on the extracted temperature-for-estimation data (dT1(t) dT2(t)); and the processor estimates the temperature of the heat source (Tb(t)) further based on the obtained thermal resistance parameter. a first temperature sensor configured to measure a first temperature (T1(t)) of a first part of a target that has a heat source; a second temperature sensor configured to measure a second temperature (T2(t)) of a second part, the second part being farther from the heat source than the first part; and a processor, wherein the processor obtains temperature-for-estimation data (dT1(t), dT2(t)), the temperature-for estimation data (dT1(t), dT2(t)) being time change quantities regarding the first temperature (T1(t)) and the second temperature (T2(t)), a thermal model includes the heat source, the first part, and the second part, in the thermal model, a first thermal resistance (Rb) represents a relation of temperatures (Tb(t), T1(t)) and heat flow (lb(t)) between the heat source and the first part, and a second thermal resistance (Rs) represents a relation of temperatures (T1(t), T2(t)) and heat flow (Is(t)) between the first part and the second part, and based on the measured first temperature (T1(t)) and the second temperature (T2(t)), the processor estimates the temperature of the heat source (Tb(t)) as a deep-body temperature of the target; characterized in that the processor extracts temperature-for-estimation data (dT1(t), dT2(t)) that are measured by the first temperature sensor and the second temperature sensor, respectively, and meet a predetermined certain condition, the certain condition corresponding to a certain state of change in a temperature of the heat source (Tb(t)), based on the obtained temperature-for-estimation data (dT1(t), dT2(t)); the processor obtains a thermal resistance parameter that represents a relation between the first thermal resistance (Rb) and the second thermal resistance (Rs) in the thermal model based on the extracted temperature-for-estimation data (dT1(t) dT2(t)); and the processor estimates the temperature of the heat source (Tb(t)) further based on the obtained thermal resistance parameter.