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In this paper, analysis and design method of a metal-insulator-metal (MIM) structure is introduced. When an electromagnetic wave enters an MIM structure, a surface plasmon is excited on both metals. In this case, an electromagnetic-field standing wave by the surface plasmon used as a wave source is generated in the insulator. This electromagnetic-field standing wave has various modes depending on the thickness of the insulator. In this paper, we establish by calculation, 3-D simulation, and measurement that a plasmon resonant wave can be controlled by changing the thickness of the insulator. We propose a sensor with an MIM structure that exploits this phenomenon. If the insulator of the MIM structure comprises air, the target solution can be injected into the insulator by capillary action, and the MIM structure is thus used as a sensor. This is because the wavelength of the electromagnetic-field standing wave in the insulator is made shorter in proportion to the refractive index of the target solution. We have confirmed this fact by calculation, 3-D simulations, and measurement. Our measurements show good agreement with the calculations and 3-D simulations, indicating that our MIM sensor is able to detect differences in refractive index of the order of 0.7%.