A smart window control device configured to control transmittance of a smart window (120_11 to 120_15) providable as a window of a space where a temperature is controllable by an air conditioning system in accordance with a set temperature, the smart window control device comprising: an obtaining unit (803, 804) configured to obtain predicted values of external environment information about outside of the space in an interval between a first time and a second time in response to determining that the air conditioning system is stopped, the second time being when a predetermined time period has elapsed from the first time and when the air conditioning system restarts, and the external environment information being an outdoor temperature and outdoor solar irradiance; a calculating unit configured to calculate transition data of the temperature of the space in the interval based on the predicted values of the external environment information in the interval and a temperature transition prediction model, the temperature transition prediction model representing a correspondence relation between transition data of an outdoor temperature actually measured in a past predetermined time range, transition data of outdoor solar irradiance actually measured in the past predetermined time range, and actual transition data of the transmittance of the smart window (120_11 to 120_15) in the past predetermined time range; and transition data of an indoor temperature actually measured in the past predetermined time range, and the transition data indicating data values with respect to time; and a transmittance control unit configured to control the transmittance of the smart window (120_11 to 120_15) to minimize a difference between the set temperature and a temperature obtained based on the calculated transition data of the temperature. an obtaining unit (803, 804) configured to obtain predicted values of external environment information about outside of the space in an interval between a first time and a second time in response to determining that the air conditioning system is stopped, the second time being when a predetermined time period has elapsed from the first time and when the air conditioning system restarts, and the external environment information being an outdoor temperature and outdoor solar irradiance; a calculating unit configured to calculate transition data of the temperature of the space in the interval based on the predicted values of the external environment information in the interval and a temperature transition prediction model, the temperature transition prediction model representing a correspondence relation between transition data of an outdoor temperature actually measured in a past predetermined time range, transition data of outdoor solar irradiance actually measured in the past predetermined time range, and actual transition data of the transmittance of the smart window (120_11 to 120_15) in the past predetermined time range; and transition data of an indoor temperature actually measured in the past predetermined time range, and the transition data indicating data values with respect to time; and a transmittance control unit configured to control the transmittance of the smart window (120_11 to 120_15) to minimize a difference between the set temperature and a temperature obtained based on the calculated transition data of the temperature.