A transmission line includes first, second, and third signal lines defining a parallel portion. No conductor connecting the first ground conductor and the second ground conductor is between the first signal line and the second signal line, and the first signal line is closer to the ground connection conductor than the second signal line. A closest frequency difference between a fundamental wave of one of the first signal and the second signal and a fundamental wave or a higher harmonic wave of the other of the first signal and the second signal is equal to or larger than a closest frequency difference between a fundamental wave of one of the first signal and the third signal and a fundamental wave or a higher harmonic wave of the other of the first signal and the third signal.