An amplifier unit suitable for a wide variety of uses, which can flexibly support diverse sound recording scenarios, and a mobile electronic device provided with the amplifier unit. An amplifier unit has microphone/line input terminals respectively provided on the topside surface and the left side surface. When a connector is connected to a mobile electronic device, stereophonic sound is recorded using one of the terminals when the mobile electronic device is held in portrait orientation, and using the other terminal when the mobile electronic device is held in landscape orientation. It is possible to switch each of audio signals coming through the two terminals between monophonic and stereophonic.