Gravitational waves were detected in 2015 for the first time in the history of humankind. Since then, around ten gravitational wave signals coming from black hole/neutron star binary coalescences have been detected. Among various expected gravitational wave sources, the beginning of the universe is one of the most exciting targets. The inflation theory was developed to solve various problems of the big bang model such as the horizon problem. According to the inflation theory, the primordial gravitational waves produced during the inflation period could reach us. Deci-hertz Interferometer Gravitational Wave Observatory DECIGO), is the future Japanese mission, which aims at detecting the primordial gravitation waves between 0.1 Hz and 10 Hz as well as obtaining other important sciences. DECIGO has a 1,000-km arm length and Fabry-Perot cavities implemented to improve the sensitivity. DECIGO employs drag-free spacecraft in a heliocentric earth-trail orbit. We plan to launch B-DECIGO first to demonstrate technologies necessary for DECIGO, as well as to detect a variety of gravitational waves. Then we plan to launch DECIGO to detect the primordial gravitational waves.