The amount of carbon dioxide absorbed by plants grown in a field can more accurately be estimated based on environmental information of the field. A plurality of types of mutually-differing environmental information are measured regarding an installation field. For the plurality of types of environmental information, the correspondence relationship between each type of environmental information and a carbon dioxide (CO.sub.2) absorption amount is stored. A plurality of CO.sub.2 absorption amounts are acquired by referring to the respective correspondence relationship for each type of measured environmental information. The acquired plurality of CO.sub.2 absorption amounts are compared with each other and the minimum value thereof is selected as the estimated value. Information regarding the selected estimated value is displayed.