The electrical conductivity of the Earth's outer core is an important physical quantity controlling the hydromagnetic process within the liquid outer core which is responsible for the creation and maintenance of the Earth's magnetic field. The conventional method, based on electromagnetic induction by external magnetic field variations is not applicable to the core because of their insufficient penetration depth. Here we propose an alternative method based on non-dipole magnetic field data, from which fluid velocity fields in the outer core are estimated, and show that a lower bound can be imposed on the electrical conductivity of the outer core from the consideration of the possibility of dynamo action sufficient to maintain the Earth's magnetic field. The conclusion is that the electrical conductivity is larger than about 1¿105 S/m. This conclusion is consistent with the results of recent experiments made for possible core material. ¿ American Geophysical Union 1988