The patient, a six year old boy was referred to the Pedodontic Department of Meikai University for a detailed dental examination of submerged of primary teeth. The oral findings through dental examination revealed that submerging of eight primary molars and severe attrition of the upper front primary teeth were occurred. According to radiographic examination, the cause of the submerged teeth was diagnosed as being induced by ankylosis. The masticatory dysfunction was treated with in overdenture and Electromyograms were recorded in order to evaluate changes in the masticatory function before and after treatment. The occlusal contact area of the total tooth and the occlusal force were also measured with Dental Prescal®. By having the patient wear an overdenture, the masticatory rhythm, synchronization on clenching, and the EMG activity of the masticatory muscles of the patient became better balanced. Silent periods during tooth tapping were noticed. The occlusal contact area and occulsal force showed an increse of six or seven times before using the overdenture.