Radio-frequency currents vary phases of the radio-frequency voltages appearing at both positive and negative input terminals of an operational amplifier depending on inductance and resistance of conductors forming an electronic device. If the phases of radio-frequency voltages appearing at both positive and negative input terminals of the operational amplifier are different, an offset voltage of the operational amplifier varies to change the operation point of the electronic device resulting in malfunction thereof. In a circuit board for electronic device including a conductor layer for circuit portion formed on an insulated circuit board, the electromagnetic interference preventing function can be improved by forming a conductor not connected electrically to anywhere, a part of conductor connected, via a connection layer, to the positive input signal of the operational amplifier of a circuit portion allocated on the conductor layer for circuit and formed of a monolithic IC, and a part of conductor connected to the negative input signal of the operational amplifier.