Microbial degradation of a vulcanized natural rubber material treated by a wood decay fungus, Trichaptum abietinum TUES-023 was observed by X-ray CT with non-destructive 1.25 m resolution. After treatment with T. abietinum for 172 days at 25 C in a static condition, the rubber surface distorted and the inside structure became porous due to elution of a filler, calcium carbonate from the rubber. The structural change was observed from subsurface to 328 m depth in the rubber. In this study, we demonstrate that X-ray CT have an advantage as a tool to monitor structure of rubber inside without addition of physical damage to the rubber materials.