We have measured the resistivity, the thermopower, and the specific heat of the weak ferromagnetic oxide CaRu0.8Sc0.2O3 in external magnetic fields up to 140 kOe below 80 K. We have observed that the thermopower Q is significantly suppressed by magnetic fields at around the ferromagnetic transition temperature of 30 K, and have further found that the magneto-thermopower Delta Q(H, T) = Q(H, T) - Q(0, T) is roughly proportional to the magneto-entropy Delta S(H, T) = S(H, T) - S(0, T). We discuss this relationship between the two quantities in terms of the Kelvin formula, and find that the observed Delta Q is quantitatively consistent with the values expected from the Kelvin formula, a possible physical meaning of which is discussed.