The present invention provides a liquid storage tank, an in-liquid observation instrument, and an optical film that are excellent in visibility and durability and easy to clean. The present invention is a liquid storage tank having a transparent wall, and the liquid storage tank covers a first moth-eye layer having a moth-eye structure and a moth-eye structure of the first moth-eye layer on an inner surface of the transparent wall. A liquid storage tank comprising layers in this order from the transparent wall side. Preferably, the liquid storage tank includes a second moth-eye layer having a moth-eye structure on the outer surface of the transparent wall, and the second moth-eye. The layer is provided in a region facing the first moth-eye layer.本発明は、視認性及び耐久性に優れ、かつ洗浄が容易な貯液槽、液中観察器具及び光学フィルムを提供する。本発明は、透明壁を有する貯液槽であって、前記貯液槽は、前記透明壁の内面に、モスアイ構造を有する第一モスアイ層と、前記第一モスアイ層のモスアイ構造を被覆する保護層とを前記透明壁側からこの順に備える貯液槽であり、好適には、前記貯液槽は、前記透明壁の外面に、モスアイ構造を有する第二モスアイ層を備えるとともに、前記第二モスアイ層は、前記第一モスアイ層に対向する領域に設けられる。
