The thermodynamic suppression effect of cavitation arising in a NACA0015 single hydrofoil is experimentally investigated in water at mainstream temperatures of T∞ = 20 to 140°C in the present study. The cavity length at T∞ = 140°C is shorter than that at T∞ = 20°C at constant cavitation number for all cavity patterns from inception to supercavitation. On the other hand, the cavity length at T∞ = 80°C is slightly shorter than that at 20°C in a certain region in which unsteady sheet-cloud cavitation occurs. This indicates that the thermodynamic suppression effect appears easily in unsteady cavitation. In addition, the temperature reduction inside cavities in water is accurately measured using thermistors, which is inserted from the sidewall directly into the cavity. The temperature measurement is performed at a mainstream temperature of less than 80°C due to limitation of calibration for the sensor. The temperature reduction at 140°C is then predicted from the measured cavity length. It is shown that the temperature reduction inside the cavity is approximately ΔT = 0.3°C at T∞ = 80°C and ΔT = 0.05°C at T∞ = 20°C under supercavitation conditions. The predicted temperature reduction inside the cavity is ΔT = 1.1 K at T∞ = 140°C under supercavitation conditions. Finally, Fruman's prediction equation for ΔT is examined by fitting to the measured and predicted ΔT values with assuming a volume coefficient of evaporation CQ as a fitting parameter.