In order to detect a gravitational wave, a kilometer-scale interferometer with a high-power, single-frequency laser with extremely low noises is required. In the Japanese gravitational wave detector named Large-scale Cryogenic Gravitational wave Telescope (LCGT), a single-frequency laser with an output power of more than 100 W will be used. The laser system used in LCGT consists of a master laser, an amplifier, a pre-modecleaner, a modulation system, and a modecleaner. The system also includes stabilization systems for laser intensity and frequency. We have already developed most of them. This article describes the characteristics of the laser system used in LCGT and the current status of our developments.