Disclosed is an apparatus and method for signal detection according to frequency bands used in an RFID system. The apparatus includes a low-noise amplifier for low-noise amplifying a received signal, a receiver unit for detecting a CW signal and a tag signal, a transmitter unit for frequency up-converting and outputting an input signal, a PLL for generating a frequency signal according to a transmission signal band or a reception signal band, a receiver unit switch for connecting the receiver unit with the low-noise amplifier or a directional coupler, an antennas switch for connecting the antenna with the directional coupler or low-noise amplifier, and a controller unit for controlling a frequency of the frequency signal generated by the PLL, turning off transmission power in a case of an LBT scheme, controlling the receiver unit switch and the antenna switch, and determining if a corresponding channel is an idle channel.