A single circuit-configured tuner is capable of selectively switching between two operating modes. A radio frequency signal, quadrature-modulated by a base-band digital signal, is captured into a single input terminal and then split into two signals. Each signal is fed into a mixer at each branch circuit block. Each mixer receives an oscillating frequency of a local oscillator. At this time, the oscillating frequency determines the operating mode. In the first operating mode, as the oscillating frequency is determined to be equal to the rf signal, the rf signal undergoes quadrature detection, resulting in that the original base-band digital signal is obtained at an output terminal coupled to each mixer, that is, the tuner works as an I/Q detection tuner. On the other hand, in the second operating mode, as the oscillating frequency is determined so that the difference between the oscillating frequency and the frequency of the rf signal is to be equal to an IF, the tuner works as an IF tuner.