A document operation unit operates on a plurality of documents in response to a user's operation request. A document relation data acquirement unit acquires relation data among the plurality of documents. The relation data includes an identifier of the user, a date of an operation, a kind of the operation, and a task as an operation purpose. A document relation data memory stores the relation data. An indication unit indicates a task in response to a user's indication request. A document evaluation unit retrieves the relation data including the indicated task from the document relation data memory, and assigns a value to each document based on the retrieved relation data. A document presentation unit presents at least one document having a larger value.