A Cavity Ring-Down (CRD) system was applied to measure the density of negative hydrogen ion (H-) in vicinity of extraction surface in the H- source for the development of neutral beam injector on Large Helical Device (LHD). The density measurement with sampling time of 50 ms was carried out. The measured density with the CRD system is relatively good agreement with the density evaluated from extracted beam-current with applying a similar relation of positive ion sources. In cesium seeded into ion-source plasma, the linearity between an arc power of the discharge and the measured density with the CRD system was observed. Additionally, the measured density was proportional to the extracted beam current. These characteristics indicate the CRD system worked well for H- density measurement in the region of H- and extraction. [ABSTRACT FROM AUTHOR]