Steelmaking slag is an attractive material because it generally contains iron oxide and free lime, which makes it able to be regarded as flux-containing low-grade iron ore for steel production. In this study, tar vapor from the pyrolysis of pine sawdust was infiltrated within porous slag and carbon deposition occurred on the pore surface by chemical vapor infiltration. For preparation, the slag sample was dehydrated in an electric furnace to decompose hydrates in the sample, creating pores. Pine sawdust was charged from bowl feeder with flowing nitrogen gas into the reactor, where it was pyrolyzed at 500 degrees C, rapidly producing fuel gases, tar vapor, and char. Tar vapor was introduced into the dehydrated slag and trapped inside it, where tar decomposed and carbonized within the pores. The product distribution was analyzed after the experiment. Experiments were repeated by changing the temperature and time to obtain an optimum condition for this process. The purpose of this research is to examine the amount of carbon deposited within the steelmaking slag by this tar-carbonization process. The product of this process, which is carbon-containing slag, is useful for energy reduction in steelworks.