Laser-ultrasonic is one of the non-contact and non-destructive inspection methods for hot materials, complicated shapes and minute materials. It has possibilities to measure dimensions and inspect metallographic structures and defects. It is difficult to measure or inspect them by traditional methods such as an ultrasonic diagnostics. Though laser-ultrasonic can be applicable to many industrial fields, we have studied the practical measures to apply the technology to a thickness measurement system for hot materials, such as robustness of optical systems and safety measures for high-power laser. The thickness measuring techniques for cold and hot bulk steels using laser-ultrasonic were tested in our laboratory. The measuring accuracy was within 2% for cold bulk steels and 4% for hot bulk steels (compared with measured value by gauges or X-ray system). Then an experimental system has been installed in a hot rolling process line and verified in terms of the measuring accuracy, safety counter-measures and durability for high temperature or dust. As the results of this experiment, the measuring accuracy is within 4% compared with X-ray system, dust on optical devices was free due to air curtains and an air jet nozzle for more than 5 months and laser reflection are shielded completely with fireproof metallic fiber curtains.