A radio signal receiver in which a RF-receiving unit 12 converts a radio received signal to a baseband signal, an AD-conversion unit 15a, 15b converts that baseband received signal to a digital signal, and a baseband-processing unit 16 uses the digital signal to perform specified baseband processing, and in which an average-interference-level-measurement unit (SIR measurement unit) 17 measures the average interference level of the received signal, an amp-gain-adjustment unit 18 sets the gain so that a fixed binary value becomes that measured average interference level, and a variable-gain amp 14a, 14b controls the reception level of the received signal by that set gain.