Abstract: An air-conditioning and ventilation simulator for predicting environmental conditions in a subsurface space, which includes a first processing portion for obtaining the airflow velocity in branches constituting the subsurface space; a second processing portion for calculating initial values and short-term change values of wall temperature and field temperature in each of the branches, initial values and boundary values of temperature, water-vapor density and carbon dioxide concentration in each of the branches; a third processing portion for calculating the wall temperature, field temperature and airflow temperature in each of the branches; and a fourth processing portion for performing detailed calculation of the airflow velocity, temperature, humidity and carbon dioxide concentration for calculating time-varying data of the airflow temperature, water-vapor density and carbon dioxide concentration in each of the branches by using, as input data, the space structure model data, the airflow analysis result dat