本文研究日本產險公司併購重組造成市場集中、公司經營效率改變，對市場競爭程度的影響。以壟斷力指標 (Lerner Index) 衡量市場競爭程度，以赫芬達指數衡量市場集中度、以資料包絡分析法所求出之效率值衡量為公司效率。結果發現，日本產險業市場集中度及公司效率皆對競爭度有負向影響，且日本市場集中度與市場競爭度反向關係，僅存在於市場壟斷力較大的子樣本。壟斷力高之廠商，專注主要險種之產品策略、優於多角化策略。 This study assesses the impact of concentration and efficiency on competition in the Japanese non-life insurance industry and use the fixed effect model to analyze data from 2001 to 2012. We use the Lerner index as a proxy for market power while measuring market concentration by using the Herfindahl-Hirschman index. The data envelopment analysis method is also used to evaluate the efficiency of insurance companies. We find that the higher the concentration level or firm efficiency, the higher the Lerner index of the firm. Further analysis shows that the positive relationship between concentration level and market power exists only in firms with higher market power. The strategy of firms with high market power tends to be specializing in main product lines such as auto lines, but diversification can raise the market power within low-market-power firms.