A space gravitational wave antenna, DECIGO (DECI-hertz interferometer Gravitational wave Observatory) will provide fruitful insights into the universe, particularly on dark energy, the formation mechanism of supermassive black holes, and the inflation of the universe. In the current pre-conceptual design, DECIGO will be comprised of 4 interferometer units; each interferometer unit will be realized by formation flight of three drag-free spacecraft with 1000 km separation. Since DECIGO will be an extremely challenging mission with high-precision formation flight, it is important to increase the technical feasibility before its planned launch in 2024. Thus, we are planning two milestone missions. DECIGO pathfinder (DPF) is the first milestone mission for DECIGO, and key components for DPF are being tested on ground and in orbit. In this article, we review the conceptual design and current status of DECIGO and DPF.