1. A method for designing a natural laminar flow wing of a supersonic aircraft, comprising: a process, performed on a computer, of setting an initial shape of a cross-sectional wing shape; a CFD analysis process, performed on the computer, of determining pressure distribution of a flow-field in the vicinity of the cross-sectional wing shape obtained; a transition analysis process, performed on the computer, of estimating a boundary layer transition position on a wing surface; a process, performed on the computer, of setting target pressure distributions for wing upper and lower surfaces based on the pressure distribution; and a CFD-based inverse problem design process, performed on the computer, including the CFD analysis process and a shape correction process correcting the cross-sectional wing shape such that the pressure distribution obtained from the CFD analysis process converges on the target pressure distribution, wherein among the target pressure distributions, the wing upper surface target pressure distribution defines a wing chordwise direction from a wing leading edge to a wing trailing edge as a domain at each spanwise station, and moreover is provided by a functional type with parameters dependent on the spanwise station as coefficients, wherein a sensitivity of a wing upper surface boundary layer transition due to fluctuations each parameter value of the parameters is analyzed by the transition analysis process, wherein an optimal combination of parameter values that delays the wing upper surface boundary layer transition the furthest rearward at a desired Reynolds number is determined by performing a search, wherein, when the chordwise direction is an X axis, a spanwise direction is a Y axis, and a wing chord length (=c(y)) is used to make a point in the wing chordwise direction (X) from the wing leading edge at each spanwise station (Y=y) dimensionless (ζ≡x/c(y)), wherein the shape of the wing upper surface target pressure distribution at each of the spanwise stations is created such that, in an extremely narrow region from the wing leading edge in which Δζ<0.01 is established, a rapid accelerating gradient of pressure and a rapid decelerating gradient are continuous, and in a subsequent broad region in which Δζ≦ζ≦1 is established, the pressure gently accelerates while simultaneously the acceleration amount is decreased and a gently accelerating gradient asymptotically approaching a predetermined value is continuous, and wherein, among the wing upper surface target pressure distribution, for a maximum velocity attainment point of the rapid acceleration gradient at each spanwise station and a constant value of the gentle acceleration gradient, the average values of pressure distributions over predetermined ranges of the wing chord length at the same spanwise stations of an initial pressure distribution obtained by applying the CFD analysis process to the initial shape are set as the initial values of a minimum pressure value and the constant value in the transition analysis process, and then the transition analysis process is executed, and when the boundary layer transition has receded furthest rearward, both values are set as the optimum values thereof.