A new direct heating apparatus was developed for measuring the relative electrical resistivity of graphite materials in the range from room temperature to 3000 °C. The temperature dependence of the relative resistivity of isotropic high-density graphite, extruded graphite and graphite sheets showed an initial decrease to a minimum at around 600 °C followed by a linear increase up to 3000 °C. The relative error of the resistivity ratio was determined to be 1.0% on average, and 3.1% at a maximum by using a standard graphite (RM8424) certificated by NIST. For a glass-like carbon, the relative resistivity decreased with increasing temperature up to 1500 °C.