The National Space Development Agency (NASDA) started conceptual study of a space debris observation system in 1994. The project indicates Japan's intention to equip its own debris observation system as inevitable infrastructure assuming the debris problem to be serious in manned space activities in 21st century. Since 1996, NASDA has focused on a pilot radar observation system and has made a basic design of an active phased array (APA) radar to observe debris 50cm in diameter in LEO orbit. Subsequently, the space guard center project for observing space debris and near earth objects (NEO) has been promoted by the Japan Space Forum (JSF). The Japan Space Guard Association (JSGA) and NASDA have been supporting JSF to proceed with the project. The space guard center consists of Kamisaibara Space Guard Center (KSGC), which has a radar sensor, and Bisei Space Guard Center (BSGC), which has optical sensors. This paper describes overview, technologies and future concept of the radar system in KSGC.