A mobile satellite broadcasting service including an ancillary terrestrial component (ATC) takes advantage of the satellite's inherent capability to provide broadcast service over global coverage. We consider the downlink transmission concept using ATC with space-time code (STC) for the mobile satellite communication. We do not regard ATC as simply a repeater but consider it as an antenna for STC. First, transmission scenarios for an application of STC are represented. Next, we apply STC in the mobile satellite system including ATC and compare the system performance in the proposed architecture of ATC to that in the conventional structure. The simulation results are compared to the conventional downlink transmission concept for the mobile satellite broadcasting service.