Or only 25% of all eligible voters (103,959,866 voters)
Before it disappears, from Tokyo Shinbun (12/18/2012; English labels are mine):
Tokyo Shinbun comments in the article:
The current election system for the Lower House is Small Electoral District Election and Proportional Representation Election. The Small Electoral District Election is aimed at creating a two-party systems by concentrating the votes, and LDP won three times as many seats as the percentage of the votes in the total eligible voters indicates. It is an incredible dissociation from the public opinion.
The Small Electoral District Election elects 300 representatives and the Proportional Representation Election elects 180 representatives. There is only one winner in each Small District.
The Small District system is conductive to giving major, established parties more seats.
It is highly ironic that it was Ichiro Ozawa who successfully pushed for the adoption of the Small District Election format. Thanks to his system, all his followers lost in the Small District Election, who had to fight under the unfamiliar, fluffy party name of Japan Future. Mr. Ozawa, under the tutelage of Kakuei Tanaka, pushed for the electoral reform in order to create the "two-party system" for political stability. It has resulted in a huge swing, instead.
The US has the Small District Election system, but not foolhardy enough to elect all representatives all at once.