Frontal mental theta waves were recorded simultaneously with magnetoencephalography (MEG) and electroencephalography (EEG) in normal subjects. MEG analyses demonstrated that theta burst activities distribute widely in the frontal cortices of both the cerebral hemispheres during concentration on mental calculation. On the other hand, EEG showed the theta activities seemed maximal on the frontal midline part (Fz), as if they might have been generated mainly in the frontal midline cortical area.