PROBLEM: To provide a high-performance complex circuit, circuit device, circuit board, and communication device that support a wider band of frequencies.SOLUTION: A complex circuit includes a first diplexer that passes through the normal-phase signals of balanced signals and a second diplexer that passes through the reverse-phase signals of the balanced signals. A balun includes a low frequency band first balun element and a high frequency band second balun element. The first balun element and the second balun element respectively include a plurality of lines that are connected to the first diplexer and that carry signals occupying two different frequency bands and also respectively include a plurality of lines that are connected to the second diplexer and that carry signals occupying two different frequency bands. The lines form one pair of balanced lines, and the lines form another pair of balanced lines. Furthermore, the first balun element and the second balun element each include an unbalanced line.