Countermeasures against the corrosion problems caused by seawater should be sufficiently considered at the spent fuel pool of the Fukushima Daiichi nuclear power plant. In this study, the corrosion behaviors of carbon steel in diluted synthetic seawater and the effect of a corrosion inhibition treatment were evaluated. Although the corrosion rate of the carbon steel in diluted synthetic seawater was high at the beginning of the test, it decreased to below 0.5 mm/y after 500 h. A clear effect of sodium nitrite (NaNO<sub>2</sub>) on corrosion inhibition was confirmed at a low flow rate and the corrosion rate decreased to 0.004 mm/y at a flow rate of 3.2 m/s or less. The corrosion inhibition effect of sodium pentaborate (Na<sub>2</sub>B<sub>10</sub>O<sub>16</sub>) was confirmed at velocities ranging from 0.6 to 5.3 m/s. The injection of Na<sub>2</sub>B<sub>10</sub>O1<sub>6</sub> is also effective for preventing the criticality of fuel. The corrosion rate of the carbon steel decreased gradually when the electrical conductivity of the electrolyte was progressively decreased. It was less than 0.1 mm/y when the electrical conductivity was less than 1.0 mS/m. When the electrical conductivity was progressively increased and exceeded 0.4 mS/m, the corrosion rate increased and shallow corrosion spots were produced.