In this paper, we report on our study of frontal midline theta (Fm theta) activity in human subjects, recorded during mental processes such as arithmetic calculation. The Fm theta is a 6-7 Hz rhythmic wave with a duration of few seconds. The Fm theta activity is observed in the central region at the front of the head. EEGs and MEGs of Fm theta were measured simultaneously during mental calculation, and we analyzed these waveforms based on both topographic EEG maps and magnetic fields measurements. A single dipole simulated the EEG topography adequately, but there are many other dipole models which can generate a similar EEG pattern. It is difficult to estimate the source location of the Fm theta from the EEG topography alone because the EEG technique has a certain ambiguity associated with source estimation. Therefore, we considered the spatial relationships between the sources and the patterns of EEG and MEG that were simulated. Although it is not possible to obtain a unique solution for the source location of Fm theta from the EEG data alone, the simultaneous recording of MEGs from a large scalp area may result in an unambiguous solution. We therefore conclude that the simultaneous recording of both MEG and EEG data is more useful for accurate localization, than the EEG alone.