The measurement of tooth mobility is an important factor in determining the prognosis and indicated therapy of the periodontally involved tooth. It is apparent that there is need for more scientific instrument which allows assessment of tooth mobility with reasonable speed for clinical research.The purpose of this stdy is to present the apparatus that in able to measure horizontal and vertical tooth mobility in all areas of mouth automatically, and to investigate the acuracy of the method using this apparatus.1. The apparatus consists of direct current motor, water cable relese, load cell, differential transformer and measuring head. The static force produced by direct current motor is applied to a crown through water cable release and measuring head. The load is ranged 0g to 500g.The amount of this load is recorded on X-Y recorder automatically. The excursion of crown is converted into voltoge by differential transformer, and recorded on X-Y recorder automatically.2. The accuracy of this apparatus was investigated clinically. Five molars of three subjects with clinically healthy periodontium were selected as physiological tooth mobility group. Four molars of four subjects with from mild to advanced periodontitis were selected as pathological tooth mobility group. The measurments were carried out four times at two minutes on the subjects of both group. The relative error was 5.3% on physiological tooth mobility group, and 6.9% on pathological tooth mobility group. Reproducibility was assessed on the some subjects at five different settings which interval were one week. The realtive error was 10.9% on physiological tooth mobility group, and 8.5% on pathological tooth mobility group.