The Rules for Electing a DPJ Leader:
The eligible electorate is divided into three sectors: a) registered party members and party supporters b) local officials who are DPJ members and c) Diet members.
To each sector is assigned a number of points, as follows:
a) Party members and party supporters - 300 points
Registered party members and party supporters mail in their votes to their local DPJ House of Representatives district headquarters. The candidate with the most mail-in votes from a House of Representatives district wins that district's one point. If a party headquarters fails to receive 100 total votes from supporters or party members, that district's point is forfeited.
The 300 potential points correspond to the 300 House of Representatives districts.
c) Members of the Diet - two points each
The DPJ members of the Diet from both Houses attend a joint party leader election assembly. At the assembly each member writes the name of just one of the candidates on a secret, written ballot. When the ballots are opened, each vote for a candidate is recorded as two points for that candidate.
There are currently 306 DPJ House of Representatives members and 106 DPJ House of Councillors members, meaning that a potential 824 points will be awarded through the direct election at the assembly.
The candidate with the most total points from this process wins.
That is what the rules say, until someone convinces me otherwise.
Source: DPJ website, "Daihyo senkyo kisoku"
Yes, that the URL for the rules regarding the DPJ's leadership election is written out almost entirely in English strikes me as weird and embarrassing too.