A case is here presented in which mandibular first molar was horizontally impacted under the submerged primary molar. The patient was an 8 year-old-boy referred from a general dentist through the Department of Pedodontics in this hospital, whose chief complaint was a horizontally impacted mandibular first molar. Skeletally, diagnosis revealed an Angle Class II Division 2 malocclusion. The impacted first molar was uprighted with a lingual arch and a utility arch following the extraction of the submerged primary molar. The submerged primary molar was probably ankylosed causing a locking of the first molar which rotated as the root formed. The result of rotation during forming twisted the roots of the first molar, which will make root canal treatment difficult if the pulp is infected in the future. It may concluded that dental arches should be carefully observed even in the deciduous dentition.