The invention is related to a method and apparatus for measuring the temperature of a melt 52, particularly of a molten metal, with an optical fiber 10, whereby the optical fiber is fed into the melt through a disposable guiding tube 40 and whereby the optical fiber and an immersion end 50 of the disposable guiding tube are immersed into the melt both having a feeding speed whereby both feeding speeds are independent from each other. The optical fiber 10 is partially arranged in the disposable guiding tube 40, whereby the inner diameter of the disposable guiding tube is bigger than the outer diameter of the optical fiber, whereby an elastic plug 30 is arranged at the second end opposite to the immersion end of the disposable guiding tube, whereby the optical fiber is fed through the elastic plug and whereby the elastic plug reduces the gap between the optical fiber and the disposable guiding tube. The apparatus comprises a fiber coil and a feeding mechanism for feeding the optical fiber and the disposable gui
