This paper proposes a basic method for designing a formation flight using Hill's equations of motion. This method involves a trajectory design for minimum fuel consumption during relative position change of spacecraft controlled by two impulses. The condition for minimum fuel consumption when the formation size is increased has already been examined. In this study, we consider the optimization problem to obtain the time and phase of the second impulse when the phase of the first impulse is given, during the decrease in formation size. In contrast to the optimum condition examined earlier, the optimum condition in this case is complicated. We have described the procedure to obtain the optimal solution. The results of our study show that the optimum condition determined previously is applicable to calculate the optimal cost in many simulation cases, whereas the new optimum condition is helpful to compensate the difference between them.