We aim to measure the electric dipole moment (EDM) of a diamagnetic atom Xe-129 to a size of 10 to power -28 ecm. Such the EDM search requires the improvement of the precision in frequency down to a level of 1 nHz. The previous developments of the active nuclear spin maser have improved the precision of frequency determination to about 10 nHz for a one-shot measurement. However, systematic uncertainty arises from a long-term drift in frequency originating from in the external magnetic field. A comagnetoneter using He-3 has been incorporated to the system in order to cancel out a long-term drift in the external magnetic field. The developments in the comagnetometer will be discussed in the presentation.