This paper describes problems concerning carbon balance and nutrient salt in coral reefs. Coral reefs fix CO2 in two forms of organic matters and calcium carbonates. It is reported that 10% of organic matters fixed by photosynthesis may either be buried in deposits on coral reefs or flow out into open seas. Quantification of the carbon balance in coral reefs has a problem of handling organic matters in calcium carbonate skeletons as products, and a problem related to evaluation of organic matters flown out from ecological systems. Corals provide, through building foundations at shallow depths, living organisms carrying out photosynthesis with locations abundant in quantity of light. Coral reefs are thought to accumulate nutrients in their skeletons or in the foundations for deposits. They would hold nitrogen in them through nitrogen fixation, and maintain phosphor production at high levels by retaining nitrogen-to-phosphor ratio which is relatively lower than in other ecological systems. Coral reefs provide foundations to transparent sea water with extremely small amount of phytoplankton, and favorable environment for large-size animals and algae. 21 refs., 2 figs.