In spite of the fact that business ethics has been an issue discussed long in human history, the emphasis, however, has been placed mainly on the argument that business firms should contribute to the wellbeing of the large society and their customers even at the expense of their own otherwise attainable profits. Nevertheless, with the drastic change taking place recently in the external environments, there are likewise, fundamental changes in the way how business firms are run and managed. So-called ”paradigm shift” in this regard has manifested in all aspects of business entities, such as the legitimacy of their survival, the contending forces between governance and agency, the humanizing of work, the rising entrepreneurships replacing hierarchical control within the organization, and the emerging form of network transcending relationships among independent firms. All these has converged on one thing, that is, the central importance of mutual trust between various parts concerned; without this indispensable ingredient, the whole system would collapse, at least, be in chaos. Therefore, it is contended in the article that mutual trust provides the most important ethical foundation of management under the new paradigm.