A content server transmits content data designated by a content reservation request from a DTE onto a first or a second communications circuit for storage into a DCE connected to the DTE. The content reservation request indicates a time limit in which the designated content data is to be ready in the DCE. The content server manages the designated time limit, and performs scheduling processing. During the processing, based on both the managed time limit and predetermined communications information, a transmission timing is determined which ensures that the content data is completely transmitted by the time limit, and which communications circuit is optimal. The content server transmits the content data onto the determined optimal communications circuit within the determined transmission timing. Accordingly, the data transmission system can achieve an efficient use of a communications circuit in terms of transmission bandwidth, and download data from a server at a lesser expense.