My research aims to test gravity theories and cosmology through gravitational wave observations. I focus on developing the large-scale cryogenic gravitational wave telescope KAGRA and advancing research toward the realization of the space-based gravitational wave antenna DECIGO. Additionally, I conduct experiments to search for dark matter and to explore the quantum nature of gravity using these telescopes and smaller laser interferometers. My ultimate goal is to answer the fundamental question: "What is gravity?" through experimental approaches.