To investigate the cause of high magnetic susceptibility in disk-shaped black materials (BM disks) within core samples of Hole B, the Taiwan Chelungpu-fault Drilling Project, we carried out magnetic analyses of samples from the BM disks and surrounding gouge zones. Frequency dependence of low-temperature magnetic susceptibility and thermal demagnetization of low-temperature remanent magnetization revealed lower amounts of superparamagnetic grains in the BM disks than in the surrounding gouges. Magnetic grain-size reduction by shearing is therefore not a plausible explanation for the high magnetic susceptibility of the BM disks. Thermomagnetic analyses of the gouge samples showed that above 400¿C magnetization could be increased by thermal decomposition of paramagnetic minerals, whereas analyses of the BM disks showed a much smaller increase in magnetization. Thus, formation of ferrimagnetic magnetite or maghemite by thermal decomposition of paramagnetic minerals was inferred to be the cause of the high magnetic susceptibility of the BM disks, indicating that the BM disks, but not the gouges, have experienced temperatures of at least 400¿C.