My research interests are mainly physics of the Early Universe, Inflation, Dark Energy, and Modified Gravity Theories. I have worked on the constraints on a parameter of the Effective Field Theory of Dark Energy using the Gravitational Wave propagation observed at LIGO/Virgo. Recently, I have been interested in several aspects of inflation and Primordial Black Hole, for instance, the calculation of the wavefunction of the Universe in a large field limit which affects the abundance of a Primordial Black Hole. Also, I am interested in Quantum Field Theory on a de Sitter background on which a general property of a correlator of a field can be obtained analytically without referring to a specific model.