Gravitational waves: A new window to observe the Universe

Gravitational waves, small ripples in the space time, are a new and unique observational tool for exploring unknown aspects of the universe. In the first part of my talk, I will review recent discoveries and developments of gravitational wave observations. Then I will introduce my data analysis activity in the KAGRA collaboration for exploring relic gravitational waves from the early Universe, such as cosmic string bursts and a stochastic background.