In a sensor network system comprising a sensor terminal and a management server, the sensor terminal transmits observation data acquired using the sensor, the management server has a latest data storage manager which manages the latest observation data among observation data received from the sensor terminal, a history data storage manager which manages the history of the observation data from any time when data was received from the sensor terminal to the latest observation data, and a missing data manager which manages missing history data, and compensates the missing history data based on a predetermined rule, and when a request for such observation data is received, at least one of the latest observation data managed by the latest data storage manager, and history data wherein the missing data has been compensated and managed by the history data storage manager, is output according to the type of request.