During the Edo period, Hachinohe Castle served as the residence of the Nanbu clan, the lords of the Hachinohe domain, and simultaneously functioned as the local government office. Although an analytical company reported on the soil samples from toilet remains within the castle, this study conducted a new analysis. Not only were Trichuris trichiura eggs found in Layer 21, but Metagonimus yokogawai and Dibothriocephalus nihonkaienesis eggs were also present. In Layer 20, which was initially thought to be free of parasitic organisms, T. trichiura, Ascaris lumbricoides, and M. yokogawai eggs were discovered. This paper discusses the differing results from previous studies, which demonstrate that the analytical methodology of Japanese archaeoparasitology is not yet well established, and suggests ways to improve it.