An information acquisition apparatus receives summary information and displays the outline information included in the summary information. When a document information acquisition request is given, the information acquisition apparatus determines, based on the current date and time and the validity period information included in the summary information, whether or not the document information corresponding to the received summary information is within its validity period. When it is within the validity period, the information acquisition apparatus acquires the corresponding document information from a databases in an information management apparatus. When it is outside the validity period, the information acquisition apparatus creates and transmits a communication document to an information registration apparatus that registered the document information. It is possible to easily understand the outline of the document information and promptly contact the registrant of the document information by electronic mail or other means.