The relation between the strain scaling law and the temperature scaling law in flux pinning force has been studied by measuring the temperature dependence of upper critical fields of bronze-processed Nb3Sn monocore-wires. Measurement of upper critical fields has been carried out for specimens with and without the bronze matrix by measuring the resistive transition in the temperature range from 12 to 18 K and measuring the critical current density at 4.2 K. Removing the bronze matrix, the critical temperature and the slope of the upper critical field vs. temperature curve near the critical temperature are increased. Based on these data and the electron scattering model of grain boundary flux pinning, the cause of difference in the upper critical field dependence of the volume flux pinning force between the strain scaling law and the temperature scaling law has been discussed. Furthermore, experimental formulae to describe the volume flux pinning force have been proposed when both strain and temperature are simultaneously varied.