A frequency mixer circuit comprises a first differential circuit composed of first and second transistors having common-connected emitters, and a second differential circuit composed of third and fourth transistors having common-connected emitters. A collector of the first transistor is connected to a collector of the third transistor and also connected through a first resistor to Vcc. A collector of the second transistor is connected to a collector of the fourth transistor and also connected through a second resistor to Vcc. A base of the first transistor is connected to a base of the fourth transistor, and a base of the second transistor is connected to a base of the third transistor. An emitter-grounded, fifth transistor is connected at its collector to the common-connected emitters of the first and second transistors, so as to constitute a current source for driving the first differential circuit. An emitter-grounded, sixth transistor is connected at its collector to the common-connected emitters of the third and fourth transistors, so as to constitute a current source for driving the second differential circuit. The base of the first to fourth transistors are biased with a first bias voltage, and a first high frequency signal is applied in the form of an unbalanced signal to the base of the first and fourth transistors. The base of the fifth and sixth transistors are biased with a second bias voltage, and a second high frequency signal is applied in the form of an unbalanced signal to the base of the fifth transistor.