An optical cored wire for improving the measuring of the temperature in a molten steel bath comprises an optical fiber and a cover laterally surrounding the optical fiber. The cover surrounds the optical fiber in a plurality of layers. One layer is a metal coat also called metal jacket or metal tube. An intermediate layer is arranged beneath the metal tube also called filler. The intermediate layer is formed from a thermal insulating material having a melting point in the temperature range of 1000 DEG C to 1500 DEG C, preferably at 1200 DEG C to 1400 DEG C, such that the pieces of the intermediate layer are fluid upon exposure to the molten metal temperatures and the intermediate layer, the entire intermediate layer and/or the cover laterally surrounding the optical fiber can melt immediately with immersion into or immediate exposure to the molten metal. A reliable measurement of the temperature in a molten steel bath can be achieved. The insulating material is a fabric or rope or simple bundles of E-glass fibres.