We studied a method of measuring upper critical field (Hc2) of a superconductor based on a width of ΔH = ΔB region, which appears in a superconductor that volume defects are many and dominant. Here we show basic concepts and details of the method. Although Hc2 of a superconductor is fixed according to a kind of superconductor, it is difficult to measure Hc2 experimentally. Thus, results are different depending on experimental conditions. Hc2 was otained by a theory on a width of ΔH = ΔB region, which is that pinned fluxes at volume defects are picked out and move into an inside of the superconductor when the distance between pinned fluxes is the same as that at Hc2 of the superconductor. Hc2 of MgB2 obtained by the method was 65.4 Tesla at 0 K, which is quite same as that of Ginzburg-Landau theory. The reason that Hc2 obtained by the method is closer to ultimate Hc2 is based on that Fpinning/Fpickout is more than 4 when pinned fluxes at volume defects of 163 nm radius are depinned, which means that the Hc2 is less sensitive to fluctuation. The method will help to find the ultimate Hc2 of volume defect-dominating superconductors.