A new method for the simultaneous measurement of the effective ionic conductivity and effective electronic conductivity in a porous sheet consisting of an electronic conductor and an ionic conductor (electrolyte) is proposed. In this method, two potentiostats and a voltage source are used, and these instruments are connected to the sample via two terminals made of the electronic conductor and two terminals made of the ionic conductor. The underlying principle of this technique does not include any assumption about charge-transfer resistance at the ionic conductor/electronic conductor interface, since the measurement is designed to let the potential difference at the interface be the OCP value at each point throughout the sample. The validity of this method was confirmed by measuring a pseudo-catalyst layer for a proton-exchange membrane fuel cell (PEMFC). The results revealed that the effective ionic and electronic conductivity depends on the relative humidity.