Abstract: A frame body for a redox flow battery is provided with a window. The expressions A>C, B>D, and (B/A)?0.2 are satisfied, where A represents the length of a long side of a rectangle that envelops the window, B represents the width of a horizontal frame corresponding to the long side, C represents the length of a short side of the rectangle, and D represents the width of a vertical frame corresponding to the short side.

Abstract: A moisture detector includes a light-receiving element including an absorption layer having a pn-junction, or an array of the light-receiving elements, wherein the absorption layer has a multiquantum well structure composed of a Group III-V semiconductor, the pn-junction is formed by selectively diffusing an impurity element into the absorption layer, and the concentration of the impurity in the absorption layer is 5×1016/cm3 or less. The moisture detector receives light having at least one wavelength included in an absorption band of water lying in a wavelength range of 3 ?m or less, thereby detecting moisture.