A content sharing system and content importance level judging method are provided.The content sharing system provides an importance level of arbitrary content is individually judged with respect to users and a notice message valuable for a user is appropriately notified to a client or the outside tool. Further, an access user is able to set the disclosure level of access record of a user accessed the content.In the content sharing system, an importance level of content of multimedia data on a server shared with clients of a plurality of users is judged, in which the importance level of the content with respect to each user is individually calculated by interpreting existing data or a user environment, the client condition is judged, a notice rule setting is changed in accordance with the client condition or an instruction from the client, and a notice message is issued to an appropriate address based on the calculated importance level and a notice rule of the content.