There were 17 prime ministers in Showa period and 16 in Heisei period. This page is about the discription of each of them.

Post-War Period（Naruhiko Higashikuni to Ichiro Hatoyama）
Economic Miracle（Tanzan Ishibashi to Kakuei Tanaka）
Stable Economy and Bubble Burst（Takeo Miki to Toshiki Kaifu）
Heisei Recession（Kiichi Miyazawa to Yoshiro Mori）
Izanami Expansion（Junichiro Koizumi to Taro Aso）

Regime Change in 2009

Democratic Party Regime（From Yukio Hatoyama）

The Reason the Prime Ministers Switch in Short Time
Ryutaro Hashimoto and Junichiro Koizumi are the only prime ministers since Yasuhiro Nakasone to serve for more than 2 years. In Japan, parties have “intraparty factions” within party that has different opinions and leaders. No matter how much leadership he has, he must get support from all of the factions to serve for long time.
Also, Japan holds more elections than other nations, so winning elections has naturally become the goal of politicians, not how to lead the country. These are the reasons that make the changes in prime ministers more frequently. Besides, the Diet has too much power. In US, the President works independently from the Congress, but the Diet has enough power to boot the prime minister. This unbalanced system forces the prime minister to have weaker political power.

　The Change of Regime in 2009

Since 2006, the prime ministers have been replaced by Shinzo Abe, Yasuo Fukuda, and Taro Aso within 3 years after Junichiro Koizumi’s cabinet resigned with the expiration of the term. There were mainly 3 major reasons the Liberal Democratic Party (LDP) suffered a crushing defeat in 2009.

1. Sudden resignation of both Abe and Fukuda
2. Gaffes and unclear goal made by Aso
3. A disruption during the Tokyo Council Election

Both Abe and Fukuda resigned as their cabinets encountered hardship. This may have brought thoughts such as “Why do the prime ministers resign to renounce their responsibility and leave everything to other person?” These events made the LDP lose credibility with the voters.

In 2009’s Election, the LDP issued messages like “Leave it on us since we’ve been managing Japan for long time,” not “How to change Japan.” This may have caused the voters with no particular party to support to vote for the Democratic Party of Japan, instead of LDP.