The Measuring instrument of carbon dioxide concentration in seawater (pCO(2)) is developed. The instrument consists of an equilibrator, a non-dispersive infrared gas analyzer (NDIR), a carrier gas generator and a processing signal generator. The equilibrator has a cross section of 3 c㎡ and is 15 cm tall. The principle of pCO(2) measurement is based on the equilibration of a carrier gas phase with a seawater sample and subsequent determination of the carbon dioxide concentration in the carrier gas. The carrier gas circuit of the NDIR is opened to the atmosphere to maintain the carrier gas line at a barometric pressure. The present instrument can measure pCO(2) within the measuring error of about 2 ppm with sample water of about 180ml every 20 min.