A cause-and-effect relation between plasmon-resonance absorption and surface wave in a metal sinusoidal grating is investigated. Employing mode-matching method with modal expansion, we solve the problem of plane-wave diffraction by the grating and create an equivalent impedance boundary value problem on the fictitious surface which is set in the hollow of the grating. The surface wave is estimated so that the wave impedance of the wave matches the surface impedance on the fictitious surface.%金属でできた正弦波状格子に平面波が入射したときの散乱問題を，モード整合法により数値解析し，格子による散乱問題と等価なインピーダンス境界値問題を設定して，インピーダンス境界の表面を伝搬する表面波を推定することにより，共鳴吸収が起こる入射角と表面波の関係について解明する．
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