We experienced a case of odontoma which appeared in the molarregion of left mandible and caused delayed eruption of the mandibular left firstpermanent molar. The tumor aggregate surgically excised was examinedclinicopathologically with respect to the appearance, conditions of the adjacentteeth, radiological findings, histopathological findings, postoperative courseand others. Findings of excised aggregate:1) The small masses excised weighed a total of 4.835 g. Each small mass wasyellowish brown, with an uneven surface covered by a membrane.2) A radiograph showed radiopaque images corresponding to the aggregate ofthe small masses.3) Histopathological examinations revealed a mass of dental hard tissue in anirregular arrangement. The mass was composed mainly of dentin with aclear tubular structure. The dentin was surrounded sporadically by enamel,cement and a small amount of pulp tissue, resulting in a diagnosis ofcomplex odontoma.In our case, no relapse has been observed, and the prognosis is excellent. Thebulge in the molar region in left side of the·mandible has disappeared, and theeruption and root formation of the first permanent molar are in progress.