The privatization of Japan’s postal saving system has been a politically charged issue since it first started being debated in the late 1980s, and yet it provides a useful setting in which political economy of economic policy-making can be investigated empirically. Analyzing the pre-election survey of the House of Representatives candidates in 2003 and also the voting patterns of Liberal Democratic Party (LDP) members on a set of postal privatization bills in 2005, this paper asks why some politicians fiercely opposed (or supported) privatization. The econometric results show that multiple factors are important: the private interests of local postmasters and postal workers, politicians’ fundamental ideologies on the size and role of government, party disciplines and factional affiliations within the LDP. Legislative behavior on postal privatization in Japan, thus, turns out to be one case in which legislative behavior is more complex than any single theory predicts.