Abstract: Provided is an optical body capable of arbitrarily and quickly controlling the optical characteristics of incident light. A refractive index variable layer (8) formed of PLZT or other material and a magneto-optical material layer (9) formed of garnet or other material are provided side by side between a first reflective layer (3) and a second reflective layer (5). If linearly polarized light is made incident from the side of the first reflective layer (3), the incident light interacts with the magneto-optical material layer (9) and is converted into a right-circularly polarized light component and a left-circularly polarized light component. A very small retardation occurring between both the right- and left-circularly polarized light components is amplified through multiple reflections between the pair of reflective layers (3, 5) and is controlled according to a controlled refractive index of the refractive index variable layer (8).