A portable electronic unit has a transmission-side section that transmits a signal and a receiving-side section that receives a signal from the transmission-side section. The receiving-side section includes a gain control amplification device that amplifies and outputs a signal from the transmission-side section. The gain control amplification device has a first amplification circuit that amplifies and outputs a signal from the transmission-side section, a detection circuit that detects and outputs a signal from the first amplification circuit, a gain control amplification circuit that amplifies and outputs a signal from the detection circuit, and a gain control circuit that controls a gain in the gain control amplification circuit in response to a level of a signal from the detection circuit.