An observation method comprising: preparing a specimen including, as a mark, a plurality of metal particles in which localized surface plasmon resonance is excited by irradiation with light; acquiring an optical microscope image by photographing the specimen with an optical microscope; acquiring an electron microscope image by photographing the specimen with an electron microscope; acquiring information of the positions of the metal particles and information of the color of scattered light form the metal particles due to localized surface plasmon resonance in the optical microscope image; acquiring information of the positions and the particle diameters of the plurality of metal particles in the electron microscope image; determining information for associating the optical microscope image and the electron microscope image based on the information of the positions and the colors of the plurality of metal particles acquired from the optical microscope image, and the information of the positions and the particle diameters of the plurality of metal particles acquired from the electron microscope image; performing alignment between the optical microscope image and the electron microscope image based on the information for associating the optical microscope image and the electron microscope image, and forming a single image by superimposing the optical microscope image on the electron microscope image; estimating the particle diameter of the metal particles from the information of the color of the scattered light from the metal particles due to localized surface plasmon resonance in the optical microscope image; finding metal particles corresponding between the optical microscope image and the electron microscope image by checking information of the positions and the estimated particle diameters of the metal particles in the optical microscope image against the information of the positions and the particle diameters of the metal particles in the electron microscope image; obtaining information of difference in magnification between the optical microscope image and the electron microscope image, and information of difference in direction between the optical microscope image and the electron microscope image from at least two pairs of corresponding metal particles; equalizing magnifications of the optical microscope image and the electron microscope image based on the information of the difference in magnification between the optical microscope image and the electron microscope image; and aligning the optical microscope image and the electron microscope image to the same direction based on the information of the difference in direction between the optical microscope image and the electron microscope image. preparing a specimen including, as a mark, a plurality of metal particles in which localized surface plasmon resonance is excited by irradiation with light; acquiring an optical microscope image by photographing the specimen with an optical microscope; acquiring an electron microscope image by photographing the specimen with an electron microscope; acquiring information of the positions of the metal particles and information of the color of scattered light form the metal particles due to localized surface plasmon resonance in the optical microscope image; acquiring information of the positions and the particle diameters of the plurality of metal particles in the electron microscope image; determining information for associating the optical microscope image and the electron microscope image based on the information of the positions and the colors of the plurality of metal particles acquired from the optical microscope image, and the information of the positions and the particle diameters of the plurality of metal particles acquired from the electron microscope image; performing alignment between the optical microscope image and the electron microscope image based on the information for associating the optical microscope image and the electron microscope image, and forming a single image by superimposing the optical microscope image on the electron microscope image; estimating the particle diameter of the metal particles from the information of the color of the scattered light from the metal particles due to localized surface plasmon resonance in the optical microscope image; finding metal particles corresponding between the optical microscope image and the electron microscope image by checking information of the positions and the estimated particle diameters of the metal particles in the optical microscope image against the information of the positions and the particle diameters of the metal particles in the electron microscope image; obtaining information of difference in magnification between the optical microscope image and the electron microscope image, and information of difference in direction between the optical microscope image and the electron microscope image from at least two pairs of corresponding metal particles; equalizing magnifications of the optical microscope image and the electron microscope image based on the information of the difference in magnification between the optical microscope image and the electron microscope image; and aligning the optical microscope image and the electron microscope image to the same direction based on the information of the difference in direction between the optical microscope image and the electron microscope image.