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We proposed an analysis method for plasma in an electrodeless discharged lamp light-emitting by inductively-coupled plasma, and a design of a power coupler was investigated by focusing on the electric power from the exciting coil to the plasma. We found that the power of the plasma was increased and of an Al stage was decreased with moving the core upward. This result suggests that the moving of the core upward is effective to reduce wasted power loss by eddy current and improves the lamp efficiency. In order to verify the result, we changed the position of the power coupler in the commercial lamp. Resultantly, the luminous flux of the lamp was increased by approximately 150 lm (lumen), when the power coupler was moved upward by 25 mm compared with the position of a commercial lamp. Therefore, we can conclude that our proposed method is applicable to the design of the lamp and suitable design is important to obtain high lamp efficiency.