This paper introduces an instrument to measure CO2 partial pressure in sea water being developed for coral reef site measurements. The instrument uses a method that sea water is contacted with a gaseous phase via a gaseous body permeating membrane, and CO2 partial pressure in the gaseous phase is measured when CO2s in the sea water and the gaseous phase have reached a dissolution equilibrium, using an adequate analyzer. The gaseous body permeating membrane uses a material with high CO2 permeation rates, such as porous polytetrafluoroethylene. The paper describes a system that applies a gaschromatograph process to analyzing CO2 in the gaseous phase, and an automatic measuring system that utilizes a non-discrete infrared densitometer. Basic experiments are being carried out that are intended to develop a CO2 partial pressure sensor using optic fibers. This is a device to measure CO2 concentration in sea water, in which fluorescent coloring aqueous solution is contacted with sea water via a gaseous body permeation membrane, and change in fluorescence intensity in the said solution as a result of CO2 melting into it from sea water is utilized. 3 refs., 4 figs.