An electrochemical method was applied to determine the boundary between flow and stagnant regions of a calcium carbonate suspension with yield stress, A tiny platinum electrode probe attached to a rotating arm was moved at a constant velocity in the stagnant test suspension to obtain current-voltage-velocity characteristics of the probe. The electric current generated by the electro-decomposition reaction of water was measured by changing the probe velocity and the voltage between the electrodes. With increasing voltage, the current increased gradually without showing a limiting current, and dependency on the probe velocity was observed only at low velocities. By traversing the probe from the stagnant region into the flow region and measuring spatial variations in the electric current, it was confirmed that the method is successfully applicable to determining the boundary of a flow region or cavern in a suspension fluid.