The Electro-Dynamic Tether (EDT) system is being studied and developed in some agencies or universities as one of the active debris removal applications for debris mitigation. The EDT system can provides a useful de-orbiting function. However, on-orbit validation of the tether deployment system and the electron emitting device is required. In this context, Kyushu University has started the design of a small satellite for an EDT demonstration mission. This design is based on QSAT which is been developed in Kyushu University. QSAT is a low-cost 50-kg class small satellite system and established a general-purpose satellite based on the original unit concept and the distributed processing concept. Essentially, all of the subsystem components consist of COTS (Commercial-Off-The-Shelf) units. These key features allow the low-cost development and the on-orbit demonstration. This system satisfies the piggy-back conditions of the Japanese H-IIA launcher. In this paper, we introduce the system characteristics for the EDT demonstration mission and explain the benefits of overall system design.