A partly carbonized wood trunk was discovered below the Omuroyama tephra covered by a lava, and it was dated to be 3,700qù100 y.B.P., by the benzene liquid scintillation method for radiocarbon age determination. We have investigated the circumstance of the discovery of the wood sample based on an interview of the person who collected the sample during his operation of building construction. According to him, the wood specimen was found at 1.5~2 m depth beneath the ground level when the operator removed the massive blackish lava block. This specimen was laid down covered by black sandy pebbles. The size of the wood trunk was 105cm long and 45cm and 35cm along wide and short diameter, respectively. We also revealed the burial environment of the wood to be related with the Omuroyama tephras and lava flow, based on our volcanostratigraphic observation of the surrounding area. The tephras covering the trunk were identified as the Omuroyama tephra B, C and D, which were defined by Koyano et al. (1996) around this area, and these tephras were covered by the Omuroyama lava flow II (Koyano et al., 1996). The wood trunk was mostly fresh and only its branch was burned. It can be estimated that the trunk has been preserved surrounded by ground water, since several tons of ground water gushed out when the aquifer was opened for construction. On this context, this trunk was buried at the early stage of the volcanism of Omuroyama volcano covered by the tephras, and later also covered by the Omuroyama lava II. We can not conclude that the burned blanch is ascribed to direct burning by the Omuroyama lava flow II but this 14C dating resultant is also concordant with all archaeological evidences reported.