The present invention relates to a differential amplifier and a mixer for improving the linearity. The differential amplifier circuit according to this present invention, includes first and second load stages each having a predetermined voltage value, a main differential amplifier unit having a first differential stage that forms a differential pair in such a way as to amplify a difference between a first input voltage and a second input voltage, and a constant current source, which has a predetermined current driving capability and is connected serially between a power source voltage terminal and a ground terminal, and a auxiliary differential amplifier unit having a second differential stage that forms a differential pair in such a way as to amplify a difference between a third input voltage and a fourth input voltage connected between the first load stage and a second load stage, and the ground, respectively.