Research and development using unallocated terahertz band beyond 275 GHz have become active globally. In this global trend, we have been working on the development of 300-GHz CMOS wireless transceiver. Although the unity-power-gain (fmax) of the transistor in the bulk CMOS is inferior to that of the compound semiconductor, the significance of studying the terahertz by CMOS exists in future system to be realized. This presentation will discuss the significance of the terahertz by CMOS integrated circuits, the technology used therein, and the future developed by terahertz communication.