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.