A Microgrid is a small scale power system consist of distributed small power facilities such as solar power generators, wind power generators and micro-turbine cogenerators. Because of some merits such as an eco-friendly system, good quality power supply and energy security, the Microgrid has been researched and encouraged actively in many countries. For commercialization, the effective operation for the Microgrid is very important. But because the ownership of each participant of a Microgrid is different in general, the effective operation of the Microgrid is not simple. In this paper, we introduce a power market model for the effective operation of the Microgrid, and propose an agent-based operation mechanism on a power market environment and a multiagent system for the Microgrid is designed based on the proposed mechanism. To verify effectiveness of the proposed mechanism, the multiagent system is implemented, simulated and evaluated on an operation situation using the multiagent platform.