Abstract: In an electric conductivity measurement circuit according to the invention, there are provided first and second cores for forming a closed flux loop in a liquid whose electric conductivity is to be measured, a first coil is wound on the first core, and a second coil is wound on the second core in a direction opposite to the direction of the winding of the first coil, a closed circuit includes the first and second coils, a reference resistor Rr and switch means, and the switch means is periodically turned on and off to periodically incorporate the reference resistor Rr in the closed circuit, thereby calculating the electric conductivity of the liquid by comparing a voltage induced at the second coil where the reference resistor Rr is incorporated in the closed circuit, with a voltage induced at the second coil where the reference resistor Rr is not incorporated in the closed circuit.