Rradiocarbon dates of organic bulk sediment samples, as well as animal and plant fossil samples, from Holocene coastal lowland sediments are compared to clarify characteristic differences between the dates of both samples and estimate the true age of sedimentation. The samples were collected from drilled cores obtained from two sites. An OSN-Br core was drilled at a river mouth in Kuji City, Iwate Prefecture, and consisted of marine sediments showing sea-level rise during the Holocene. Two KYD-Br cores were drilled at a small valley in Yamada Town, Iwate Prefecture, and consisted of peaty sediment and interbedded event deposits (e.g., tephra and tsunami deposits). Based on a comparison with the dates of samples collected from the same horizon or ages of tephras, radiocarbon dates of organic bulk sediment samples of the OSN-Br core showed a systematic age difference between the dates of animal and plant fossil samples and tephras. These organic bulk sediment samples were interpreted to include old carbon derived from the hinterland, particularly from Paleogene to Cretaceous sedimentary rocks. On the other hand, radiocarbon dates of organic bulk sediment samples of the KYD-Br cores agree with dates of in-situ plant fossil samples (seed and leaf) and are consistent with the age of tephras. Therefore, it can be assumed that the dates of organic bulk sediment samples represent the true age of sedimentation.