Measurement of negative hydrogen ions (H-) is one of the important diagnostics for development of negative ion sources. Cavity-Ring-Down (CRD) measurement system was successfully implemented for the H- density measurements in the Field-Effect-Transistor based Radio Frequency ion source. Reliability of the CRD system was experimentally confirmed, and basic characteristics of the H- ions and the ion source were studied. The measured H- density varied from 1015 to 1016 m-3 depending on the source parameters, such as input RF power, axial magnetic field in the driver region, and source pressure. The H- density increased linearly with increase of the RF power and the axial magnetic field increased, due to improvement of power transmission and plasma production efficiency. Source gas pressure dependence of the Hdensity showed an increase of H- density with the decrease of pressure. Higher electron temperature and lower neutral gas resulted in increased production of vibrationally excited molecules and decreased destructions of H- ions, which was feasible for in high H- density at low source pressure.