This paper proposes a new communication system called RoCoS (Room-based Communication System) that allows multiple users to communicate with each other through their virtual 3D spaces called rooms located on the Internet. This system consists of two main sub-systems, i.e., 3D Messenger and Edit Tool. 3D Messenger provides multiple users with their collaborative operable and communicable environment on the Internet. Edit Tool allows each user to create his/her own virtual 3D space, i.e., individual room, and to create any avatar represented as his/her own 3D character used in 3D Messenger. Each room provided by 3D Messenger is regarded as 3D version of a web page because each room exists separately on its dedicated computer managed by its owner (Administrator) and the owner can edit, decorate and modify his/her room as he/she wants using Edit tool. This paper describes details and clarifies the usefulness of the system by showing its several functionalities and application examples.