In order to reduce CO2 emissions in the steelmaking process, it is anticipated to use more steel scraps. With the use of steel scraps, however, tramp elements such as copper and tin can be accumulated in the steel scrap because they cannot be eliminated by slag refining. Typically, the copper content in the scrap is expected to exceed the tolerance limit by 2050. As the copper content increases, hot shortness occurs during the hot rolling process. It has been considered that liquid metal embrittlement is the dominant reaction mechanism. However, no direct observation of the grain boundary wetting of steel by liquid copper has been carried out. In this study, for the first time, direct observation of the surface and grain boundary wetting of steel plates by liquid copper was carried out with Confocal Laser Scanning Microscope (CLSM) at high-temperatures. Once liquid copper was formed, the spreading occurred rapidly isotropically. When the apparent contact angle approached the equilibrium value, the spreading of liquid copper occurred preferentially through the grain boundaries. The penetration of liquid copper through the grain boundaries was accelerated by increasing temperature. From the cross-sectional observation after experiments, it was revealed that the hot shortness was enhanced at higher temperatures.