A mixer circuit includes: a rat race circuit including a ring-shaped transmission line with a first terminal, a second terminal, a third terminal, and a fourth terminal, the first to fourth terminals being disposed, in that order, clockwise along the transmission line and equally spaced λLO/4 from one another, except that the first terminal is spaced 3*λLO/4 from the fourth terminal, where λLO is the wavelength of the LO signal applied to the mixer circuit; an LO terminal connected to the first terminal; a first diode and a second diode connected in the same polarity to the second terminal and the fourth terminal, respectively; and an RF terminal and an IF terminal both connected to the third terminal. The frequency of the LO signal is one-half of the frequency of the RF signal applied to the mixer circuit.