We investigated a method for measuring the refining degree of bamboo charcoal using an alternating current. The bamboo charcoal was made under heating conditions of 400°–900°C (set temperature) and 0—3 h (holding time at each set temperature). The qualities of the bamboo charcoal could not be estimated from the yield, and electric tests were required. The effect of the variation in sample thickness on the impedance could be ignored. Attaching two plate electrodes to the same surface of a specimen enabled high accuracy and practical use. The impedance was found to be a suitable index for estimating the refining degree of the bamboo charcoal, such as the specific electric resistance. We believe that bamboo changes from an insulator to a conductor suddenly when processed at 600°–750°C for 0–2 h. It is possible that the integral of temperature with time in a specimen during heating is useful for approximately estimating impedance.