I am a theoretical physicist in condensed matter physics. Condensed matter consists of numerous atoms; its physical properties are mainly governed by electrons in the atoms. The properties of matter are, in other words, responses of the electrons to the external stimuli, e.g., heat, light, electric/magnetic fields, and particle beams. Also, the properties can be drastically changed by the sufficiently strong stimuli, which is sometimes called a phase transition. Thus, we need to study the quantum mechanical motion of the many electrons for the comprehensive understanding of the properties of matter. It is noteworthy that an infinite variety of matter emerges from a huge number of electrons whereas each electron is indistinguishable from the others and its motion is described by the Schrödinger equation. Condensed matter physics aims at finding out and understanding the universal law that governs the properties of matter, which I would like to contribute to.