Synopsis :The progress of measurement technologies in the Japanese steel industry over the last 100 years is reviewed. In the early period, the main objects of measurement were temperature in the blast furnace, steelmaking process, etc., using thermocouples and radiation thermometers. Through the years, instrumentation technologies have been developed with progress in electronics, and various techniques, including ultrasonic, image processing, laser technology, and others, have been applied. Several remarkable in-line measurement technologies published in "Tetsu-to-Hagané" are reviewed in the history below, and the future direction is considered.