The dielectric properties of totally dried sugi (Cryptomeria japonica) wood block and powder specimens treated at various temperatures up to 800 o C were measured in the range from 20 Hz to 1 MHz and from -150 to 20 o C. Relaxation ascribed to the motion of methylol groups was detected in specimens treated at temperatures below 300 o C and its magnitude decreased with increasing temperature. The electric conductivity decreased with increasing temperature up to 400 o C. Another form of relaxation was recognized for block specimens treated at temperatures between 500 and 600 o C. Cole-Cole's circular arc law was applied to the experimental results. The relaxation magnitude was 60-75 and 24-28 in the longitudinal and radial directions of the block specimens, respectively. Identical relaxation was also observed in the powder specimen. The electric conductivity began to increase remarkably from 500 o C. These results suggested that a small volume fraction of particles with large conductivity is formed at microscopic levels in the cell walls. It is considered that this relaxation is due to interfacial polarization.