我國雙首長制的憲政運作為什麼不會像法國第五共和一樣呈現換軌的特色？本文將以法國第五共和的憲政運作為觀察參考的依據，探討我國憲政運作無法順利換軌的制度因素。 法國的換軌式憲政運作之所以能夠順暢，主要是基於以下幾個機制的輔助：第一，法國國會議員選舉所採取的兩輪投票制塑造了國會中穩固團結的左右大聯盟，迫使總統在無國會多數支持時，不敢忽視國會多數的實力；而我國立法委員選舉所採取的單記非讓渡投票制（SNTV）卻導致立法院中無法形成穩固團結的多數黨（或聯盟）。第二，法國的行政指揮系統是一元化的，因而當憲政運作換軌至總理主政時，總統沒有太多行政資源可以干預總理的施政；而我國的行政指揮系統卻是二元化的。第三，法國總統在憲法上對總理沒有主動免職權，因此可以確保在總理主政時總理只須對國會負責而不對總統負責；而我國總統在實際憲政運作上行政院院長則有主動免職權。第四，法國總統有主動解散國會的權力，使得新總統上任時可以主動塑造與自己一致的國會多數；我國總統則無主動解散立法院的權力。第五，法國總統選舉採取的是兩輪對決制，且與國會議員選舉皆是兩輪投票制，因而大幅提高新總統成功塑造與自己一致之國會多數的機率；然而我國總統選舉採取的是相對多數制，而且立委與總統選舉制度迥然不同。以上五項制度因素中，前三項因素是法國在面臨「新國會 v.s. 舊總統」的情況時，憲政運作得以順暢地由總統制換軌到內閣制、而我國卻不能的原因；後兩項因素則是法國在面臨「新總統 v.s. 舊國會」的情況時，憲政運作能夠順暢地從內閣制換軌到總統制、而我國卻不能的原因。 就此看來，換軌式的憲政運作乃是建立在總統的憲法權力、政黨體系、國會選舉制度、總統選舉制度、行政指揮建制各個面向的整體配套上，任一環節都非常重要。因此，如果我國未來的憲政體制仍然維持雙首長制的架構，並且希望如同法國呈現換軌式的憲政運作，在憲政工程上便應全面顧及確保換軌順暢的整體配套設計，而不應只是偏重其中某個面向。

The paper tries to explore why the ROC’s constitutional system, so called semi-presidentialism (deal-leadership system), cannot work like French Fifth Republic’s, “alternating” between presidential and parliamentary phase. Focusing on the institutional mechanism, the paper indicates five critical factors which result in different constitutional outcomes in France and the ROC. First, the two-ballot system for the election of the French National Assembly members has contributed to two firm political alliances in the National Assembly, while the ROC’s SNTV system for the election of legislators shapes an unstable multi-partism in the Legislative Yuan. Second, the administrative command system in france is unitary, while it isn’t in ROC. Third, the French president isn’t granted the constitutional power to dismiss the head of the cabinet at will, but the president of ROC seems to own this power, in view of the empirical constitutional exercise. Fourth, the French president can dissolve the National Assembly voluntarily, but the ROC president cannot dissolve the Legislative Yuan unless the latter passes a vote of no-confidence on the Executive Yuan. Fifth, the French presidential election adopts majority vote (runoff election), while the ROC presidential election adopts plurality vote. Among these institutional factors, the first three enable the French constitutional system to alternate from presidential phase to parliamentary phase in case that the president is not supported by the majority of the newly elected assembly. The last two help the French constitutional system alternate from parliamentary phase to presidential phase when the majority of the assembly isn’t the political alliance to which the newly elected president belongs. Due to the lack of the alternating mechanism which exists in the French system, the ROC’s constitutional system fails to alternate smoothly.