The aim of this study is to get a tester which can be made easily and has sufficient measuring accuracy for practical use. A conventional tester for external gears is utilized. Tooth profile errors are detected by an electric micrometer and recorded on an X-Y recorder. Internal gears are set on a rotary table and chucked by three pins equipped on the table from the outside of gear blanks. The centering of gears is performed simultaneously in the chucking. Even if an eccentric error of 0.05 or so exists in the centering, its influence on the tooth profile error curve can be neglected by selecting the tooth of internal gears suitably. Influences of the position error of the feeler, the inclination of the top plane of the table, and the deformation of the outside cylinder of gear blanks are examined theoretically and allowable values of these errors are given.