The system developed for the continuos temperature measurement of molten metal (1), like in the case of the continuous casting machine tundish (2), uses an optical process to control the continuous casting machine speed, and it consists of an optical infra-red sensor (8) protected by a cooled jacket (30). This two-color sensor (8), fitted with optical fiber (9) and an optical signal converter (10), is focused inside a high thermal and light conductivity ceramic tube (15), and it enables accurate temperature readings of molten steel (1) in the tundish (2).This practical device avoids the inconvenience of the method currently being used. It reduces the operator's high temperature exposure time, lowers maintenance downtime, minimizes the operating risks, improves safety and enables fast, simple replacement, resulting in improved slab quality and, as a consequence, lower costs.