Existing studies on single nontransferable voting system under multi- member district (SNTV-MMD) have focused mainly on its proportionality and consequences and paid little attention to its changeablility. When Japan did replace it with a new electoral system in 1994, most works ascribed the reform to the discontent toward corruption induced by the old system. Such a theory accounted for neither the timing nor the outcome of the reform. This article provides a different explanation by treating institutional choice as a result of strategic interaction among individual politicians. Extending a fundamental theorem in spatial theory, we propose a general proposition regarding the possibility of electoral reform: the status quo system is irreplaceable if and only if it is a median in all directions. We then ascertain two initial conditions whereby the proposition can be applied: that SNTV-MMD is roughly the median regarding the proportionality of electoral systems and that the system induces divergent preferences againist itself. Three hypotheses are then derived: (1)SNTV-MMD is irreplaceable if the reform is not associated with other issues, (2) SNTV-MMD is irreplaceable if some party controls the majority of seats, (3)whenever the status quo is irreplaceable, there is more than one viable alternative. These propositions are corroborated by the Japanese experience. Attempts to change SNTV-MMD failed until the LDP broke up and electoral reform became an indispensable issue in Japan's party realignment. The final bill also reflected the common interest of the LDP and the new parties on the single-member district system. However, the stability of the new system is affected by the same logic that this study portrays.