日本自約1990年代末期推動「平成大合併」，擬將約3200個市町村自治體整併為約1000個，以利區域廣域政策的推動，並減少地方自治體的行政營運費用。然而，自治體的存續為憲法所保障，尤其在民主時代，日本中央政府難以用強制的手段大規模進行市町村自治體的合併。因此，有些市町村在沒有行政區域合併的情況下，選擇用跨域治理的方式達成廣域治理的需要。

而為解決廣大區域的共同治理課題，實務上常以地方自治體間的「行政區域合併」或是跨域治理來因應大都市周邊地區的人口及互動關係的成長;然而合作與合併的發生的契機為何?本文將進一步探究。並以日本東京都內、外部治理，以及擴及到「首都圈」的治理運作進行分析與說明。 本研究的研究發現如下:一、日本地方自治體的合併主要發生在財政較為困窘的縣份;二、在人口密集的關東地區，因轄區內基礎自治體的財政相對穩健，難以促成合併，其廣域治理事務主要以自治體間的合作來進行; 三、合併涉及自治體的存滅，這是地方自治體最後的選擇，自治體會先評 估合作，若財政無法支持，最後才會選擇合併;四、從日本全國來看，當合併的數量越高時，合作的數量就會減少，反之亦然。

Japan has promoted the Great Heisei Mergers since the late 1990’s. Under this program, the Japanese government planned to merge approximately 3200 villages, towns, and cities into roughly 1000; thereby facilitating the promotion of regional policies and reducing the administration and operation expenditures of local autonomous bodies. However, the existence of such bodies is guaranteed by the Constitution of Japan. Particularly in the current era of democracy, the central government of Japan has experienced difficulty in forcing the large-scale mergers of villages, towns, and cities. Therefore, some autonomous bodies without administration zone mergers have chosen to practice cross-boundary governance in order to fulfill the needs for large-area governance.

To resolve problems concerning co-governance in large areas, administration zone mergers and cross-boundary governance are frequently adopted in practice between local autonomous bodies in response to the population growth and interaction of regions surrounding metropolises. However, the drive behind such collaboration and mergers merits in-depth exploration; hence, this study further explored this topic. Analysis was performed to describe the governance approach within and outside of the Tokyo Metropolis as well as that of the Greater Tokyo Area. The following results were identified: 1. Mergers of autonomous bodies mainly occurred in prefectures with poor fiscal status. 2. Autonomous bodies in the populous Kanto region exhibited relatively stable fiscal status; hence, mergers were difficult to execute in this region. Large-area governance therein was mostly performed through collaboration between autonomous bodies. 3. Mergers determined the survival of autonomous bodies and were considered the last resort of such bodies. Autonomous bodies first evaluated the feasibility of collaboration with other bodies. Mergers with other bodies only occurred when their fiscal conditions could not support their operations. 4. In regions with a large number of mergers, the number of collaborations became relatively small, and vice versa.