An indoor digital radio apparatus, IDU, for a digital radio apparatus which includes said IDU, an outdoor radio apparatus, ODU, that connects to said IDU through a cable, and an antenna, performs a point-to-point radio communication with an opposing said digital radio apparatus, and has a (1+1) switching configuration for a backup in case of failure of said digital radio apparatus or degradation of radio line quality comprising: an active modulation and demodulation unit for modulating a first input signal and demodulating a first received signal from a first outdoor radio apparatus, first ODU; a backup modulation and demodulation unit for modulating a second input signal and demodulating a second received signal from a second outdoor radio apparatus, second ODU; a shared unit for controlling said IDU, and having a connector for input/output interface connecting to a communication apparatus, via a communication cable, characterized in that said active modulation and demodulation unit is detachable and includes a first cable connection unit which connects via a first single cable to said first ODU, said backup modulation and demodulation unit is detachable and includes a second cable connection unit which connects via a second single cable to said second ODU, said active modulation and demodulation unit, and said backup modulation and demodulation unit and said shared unit are accommodated in a single IDU case, said single IDU case has a double-layer structure, and has a first and a second layer for said accommodation in a front side, wherein said first layer is arranged at the top of said IDU and said second layer is arranged at the bottom of said IDU, said active modulation and demodulation unit and said backup modulation and demodulation unit are mounted detachably to said first layer from a front side and are adapted to dissipate the heat in said IDU upward, have half a width of the shared unit and are arranged horizontally in parallel in said first layer above said shared unit, and said shared unit has a control circuit for said first ODU and said second ODU, and is mounted to said second layer in such a manner that said IDU is downsized. an active modulation and demodulation unit for modulating a first input signal and demodulating a first received signal from a first outdoor radio apparatus, first ODU; a backup modulation and demodulation unit for modulating a second input signal and demodulating a second received signal from a second outdoor radio apparatus, second ODU; a shared unit for controlling said IDU, and having a connector for input/output interface connecting to a communication apparatus, via a communication cable, characterized in that said active modulation and demodulation unit is detachable and includes a first cable connection unit which connects via a first single cable to said first ODU, said backup modulation and demodulation unit is detachable and includes a second cable connection unit which connects via a second single cable to said second ODU, said active modulation and demodulation unit, and said backup modulation and demodulation unit and said shared unit are accommodated in a single IDU case, said single IDU case has a double-layer structure, and has a first and a second layer for said accommodation in a front side, wherein said first layer is arranged at the top of said IDU and said second layer is arranged at the bottom of said IDU, said active modulation and demodulation unit and said backup modulation and demodulation unit are mounted detachably to said first layer from a front side and are adapted to dissipate the heat in said IDU upward, have half a width of the shared unit and are arranged horizontally in parallel in said first layer above said shared unit, and said shared unit has a control circuit for said first ODU and said second ODU, and is mounted to said second layer in such a manner that said IDU is downsized.