We report a conceptual study of SELENE (Selenological and Engineering Explorer) system, and the mission instruments for selenodesy using SELENE Relay Satellite which will be launched in 2003. Four-way Doppler measurements and two-way ranging measurements relayed by the Lunar Polar Orbiting Satellite and the Relay Satellite will be made to determine the precise orbits. Differential VLBI observation will also be made using radio sources on the relay satellite and the lunar surface. Analyzing the disturbances of the orbits of these spacecrafts, distributions of the lunar gravitational field will be elucidated. Simplified transponders and radio sources on the Relay Satellite are designed to accomplish minimized and lightened satellite system of about 40kg.