Electric potential difference observed by using submarine cables contains various kinds of geophysical information. In this paper, we attempt to constrain the strength of the toroidal magnetic field at the CMB by comparing the time-dependent part of cable voltages and EM responses by a forward modeling. Assuming the linear-trend of the observed electric potential reflects thirty-year period variation, which is taken as a reference time-scale for the variation, of the magnetic field at the CMB, the toroidal field strength at the CMB is estimated to be about 1--100 times that of the poloidal magnetic field there. The values should be considered as approximate upper bounds of the toroidal field variation because of short data length analyzed. Accumulation of a longer time series will lead to more accurate estimation of amplitude of time variations which enables us to give more definite constraints on the behavior of the toroidal field. ¿ 1998 American Geophysical Union