The present invention relates to a coolant composition for fuel cell coolant, comprised of sulfur containing alcohol or sulfur containing phenol having at least one sulfur atom per molecule, which effectively prevents increase of electric conductivity due to oxidation of its base component for a long time. The coolant composition when diluted with water to a predetermined concentration effectively suppresses increase of electric conductivity of the coolant and maintain fluctuation of electric conductivity within the range from 0 to 10 μS/cm so that power generation efficiency of the fuel cell may not be lowered.