A room temperature estimating device comprising: a room temperature obtainer configured to obtain room temperature data; an outside air temperature obtainer configured to obtain outside air temperature data; a storage configured to store the room temperature data obtained by the room temperature obtainer and the outside air temperature data obtained by the outside air temperature obtainer with each piece of the room temperature data and the outside air temperature data associated with a corresponding measured date and time; a prediction formula producer configured to produce two or more prediction formulae each expressing a relation between pieces of room temperature data and pieces of outside air temperature data at a specified time of day, based on the pieces of the room temperature data and the pieces of the outside air temperature data corresponding to the specified time in each of two or more days during a given extraction period, which are stored in the storage, the two or more prediction formulae corresponding to two or more specified times of day, respectively; a prediction change obtainer configured to obtain a predicted temporal change in outside air temperature; a room temperature estimator configured, based on the temporal change in outside air temperature obtained by the prediction change obtainer, to apply an outside air temperature at a target time, corresponding to one of the two or more specified times, to the prediction formula produced in relation to the one of the two or more specified times, and thereby to estimate a room temperature at the target time; and a clock configured to measure a current date and time, wherein the extraction period is included in any one of division periods which a period of one year is divided into based on climatic environment, the room temperature estimator is configured to apply, to predict a room temperature in a certain division period, a prediction formula of two or more prediction formulae produced based on pieces of the room temperature data and pieces of the outside air temperature data during an extraction period included in the certain division period, the prediction formula producer is configured to newly produce two or more prediction formulae respectively corresponding to two or more times of day, at a transition from one division period to a next division period determined based on the current date and time measured by the clock, and the room temperature estimator is configured to estimate the room temperature based on one of the newly produced two or more prediction formulae. a room temperature obtainer configured to obtain room temperature data; an outside air temperature obtainer configured to obtain outside air temperature data; a storage configured to store the room temperature data obtained by the room temperature obtainer and the outside air temperature data obtained by the outside air temperature obtainer with each piece of the room temperature data and the outside air temperature data associated with a corresponding measured date and time; a prediction formula producer configured to produce two or more prediction formulae each expressing a relation between pieces of room temperature data and pieces of outside air temperature data at a specified time of day, based on the pieces of the room temperature data and the pieces of the outside air temperature data corresponding to the specified time in each of two or more days during a given extraction period, which are stored in the storage, the two or more prediction formulae corresponding to two or more specified times of day, respectively; a prediction change obtainer configured to obtain a predicted temporal change in outside air temperature; a room temperature estimator configured, based on the temporal change in outside air temperature obtained by the prediction change obtainer, to apply an outside air temperature at a target time, corresponding to one of the two or more specified times, to the prediction formula produced in relation to the one of the two or more specified times, and thereby to estimate a room temperature at the target time; and a clock configured to measure a current date and time, wherein the extraction period is included in any one of division periods which a period of one year is divided into based on climatic environment, the room temperature estimator is configured to apply, to predict a room temperature in a certain division period, a prediction formula of two or more prediction formulae produced based on pieces of the room temperature data and pieces of the outside air temperature data during an extraction period included in the certain division period, the prediction formula producer is configured to newly produce two or more prediction formulae respectively corresponding to two or more times of day, at a transition from one division period to a next division period determined based on the current date and time measured by the clock, and the room temperature estimator is configured to estimate the room temperature based on one of the newly produced two or more prediction formulae.