A temperature measurement device to which a thermocouple is externally connected, comprising: a casing provided with a cold junction of the thermocouple; a first temperature sensor disposed in a vicinity of the cold junction inside the casing and configured to measure a cold junction temperature (Tc) of the thermocouple; and a second temperature sensor disposed at a position different from the first temperature sensor inside the casing and configured to measure an internal temperature (Tin) inside the casing, characterized in that the temperature measurement device further comprises: a control unit configured to calculate an external temperature (Tout) of the casing by applying the cold junction temperature (Tc) measured by the first temperature sensor and the internal temperature (Tin) measured by the second temperature sensor to an interpolation equation including subtracting a correction amount (Tg) from the cold junction temperature (Tc), where the correction amount (Tg) is calculated by multiplying a difference between the internal temperature (Tin) and the cold junction temperature (Tc) by a gradient and adding an intercept, wherein a measurement temperature measured at a predetermined position outside the casing and a junction temperature (Tc) and an internal temperature (Tin) corresponding to the measurement temperature are set as one set of measurement data and plural sets of the measurement data are acquired under at least two different conditions, the predetermined position is a position at which the temperature is predicted by the calculated external temperature (Tout) and is a position in the periphery of the casing, and the control unit is further configured to acquire the gradient and the intercept of the interpolation equation by performing a linear interpolation for the plural sets of measurement data using the interpolation equation. a casing provided with a cold junction of the thermocouple; a first temperature sensor disposed in a vicinity of the cold junction inside the casing and configured to measure a cold junction temperature (Tc) of the thermocouple; and a second temperature sensor disposed at a position different from the first temperature sensor inside the casing and configured to measure an internal temperature (Tin) inside the casing, characterized in that the temperature measurement device further comprises: a control unit configured to calculate an external temperature (Tout) of the casing by applying the cold junction temperature (Tc) measured by the first temperature sensor and the internal temperature (Tin) measured by the second temperature sensor to an interpolation equation including subtracting a correction amount (Tg) from the cold junction temperature (Tc), where the correction amount (Tg) is calculated by multiplying a difference between the internal temperature (Tin) and the cold junction temperature (Tc) by a gradient and adding an intercept, wherein a measurement temperature measured at a predetermined position outside the casing and a junction temperature (Tc) and an internal temperature (Tin) corresponding to the measurement temperature are set as one set of measurement data and plural sets of the measurement data are acquired under at least two different conditions, the predetermined position is a position at which the temperature is predicted by the calculated external temperature (Tout) and is a position in the periphery of the casing, and the control unit is further configured to acquire the gradient and the intercept of the interpolation equation by performing a linear interpolation for the plural sets of measurement data using the interpolation equation. a control unit configured to calculate an external temperature (Tout) of the casing by applying the cold junction temperature (Tc) measured by the first temperature sensor and the internal temperature (Tin) measured by the second temperature sensor to an interpolation equation including subtracting a correction amount (Tg) from the cold junction temperature (Tc), where the correction amount (Tg) is calculated by multiplying a difference between the internal temperature (Tin) and the cold junction temperature (Tc) by a gradient and adding an intercept, wherein a measurement temperature measured at a predetermined position outside the casing and a junction temperature (Tc) and an internal temperature (Tin) corresponding to the measurement temperature are set as one set of measurement data and plural sets of the measurement data are acquired under at least two different conditions, the predetermined position is a position at which the temperature is predicted by the calculated external temperature (Tout) and is a position in the periphery of the casing, and the control unit is further configured to acquire the gradient and the intercept of the interpolation equation by performing a linear interpolation for the plural sets of measurement data using the interpolation equation.