Tokyo Institute of Technology (Tokyo Tech) group undertake student-led development of small-satellite. Recently a number of 1kg class pico-satellites like our "CUTE-I" which has been successfully launched in 2003, are being launched by universities all over the world. However the size of pico-sized satellite is pointed out that it is too small to carry out the mission we really desire. This paper explains an examination of bus system and mission example for 20kg Class nano-satellites.