A mixer for outputting an intermediate frequency signal as the result of mixing of a radio frequency signal and a local oscillator signal which are inputted into the mixer. The mixer comprises a balanced-to-unbalanced conversion circuit and a bridge-coupled diode circuit and the balanced-to-unbalanced conversion circuit is constructed of first and second rat race circuits, one being for the radio frequencey signal and comprising a first ring-shaped line and the other being for the local oscillator signal and comprising a second ring-shaped line. The mixer is arranged to include a substrate having first and second pattern-forming surfaces and the first ring-shaped line of the first rat race circuit is formed as a pattern on the first pattern-forming surface of the substrate and the second ring-shaped line of the second rat race circuit is formed as a pattern on the second pattern-forming surface. The bridge-coupled diode circuit is positioned on the first pattern-forming surface thereof and inside the patterned first ring-shaped line.