A current source block and a negative resistance generation block are connected in parallel. The negative resistance generation block generates a negative resistance in response to the minute variations of an output voltage. Thus the output resistance of a current source circuit is given by the combined resistance of the negative resistance and the resistance of a resistor in the current source block connected in parallel. The resistance of the resistor in the current source block and the negative resistance are controlled to be substantially the same to thereby increase the output resistance of the current source circuit. The current source circuit serves to increase an output resistance when viewed from an differential output terminal. As a result, use of this current source circuit realizes a differential amplifier providing a high gain.