1994年日本改革眾議院議員選舉制度，將行之多年的「中選舉區制」，改為「小選舉區比例代表並立制」。選舉制度改革的目的在形成兩黨政治、消除派閥競爭、導正為政策本位的選舉、節省競選經費及反應多元民意等五項，當中以形成兩個主要政黨，成為可以有政黨輪替執政，建立負責任的政府的政黨體系，更是最主要的訴求。檢視自1996年至2005年四次眾議院選舉，已隱然有兩黨對抗的態勢，惟自民黨仍居優勢。日本的新選舉制度是介於相對多數決制與比例代表制間的混合制，由於小選舉區的名額佔62.5%，在其他條件不變的情況下，僅就制度面論，對形成兩黨制有加乘的作用，但保留37.5%比例代表名額，因此難以形成如英美單純的兩黨制。小黨仍有特定發展空間。本文根據「杜弗傑法則」、李普賽及雷伊對兩黨體系的定義，和薩托利對競爭性政黨體系分類中「溫和多黨體系」的內涵予以修正，假設日本採行的「小選舉區比例代表並立制」，這種介於「相對多數決制」與「比例代表制」之間的選舉制度，由於制度性的因素，有形成「不完全的兩黨制體系」的傾向，並無法建立長期有效之「兩黨制體系」運作模式。此一假設將以眾議院新選舉制度施行後之四次選舉為標的，進行分析和驗證，並在結論中說明。

In 1994, Japan reformed its Lower House’s election system and changed the Middle Election District System, which was in place for many years, to the Simple Plurality System and Proportional Representation. The five objectives of the reform were to form a two-party System, to eliminate clique competitions, to lead to policy-based elections, to reduce campaign costs, and to reflect diverse public opinions. The main focus was on forming the two-party system, so parties in power could alternate and party systems and establish responsible offices.From the review of the four Geneval Elections from 1996 to 2005, it was noted that the two-party system was starting to form, with the Liberal Democratic Party still leading. Japan’s new election system was a hybrid between the relative majority representation and proportional representation. Since 62.5% of the seats are elected from the Simple Plurality System, other things being equal, the new system facilitates the formation of a Two Party System. However, with 37.5% of the seats elected from Proportional Representation, it is difficult to form a pure two-party system as the US or the UK. Small parties still have room for development.The thesis was based on the Duverger’s Rule, L. Lipset’s and D. W. Rae’s definitions of the two-party system, and Sartori’s contents about within competitive party systems. It established Japan’s Simple Plurality System and Proportional Representation, a hybrid between the relative majority representation and proportional representation, and tended to form a Semi-Two-Party System instead of a long-term, effective two-party system.The assumption was validated by analyzing and verifying the results of the four General Elections after the adoption of the new election system.