PURPOSE: To obtain a mobile object communication unit by which information can be transmitted between a mobile object and a ground station in two directions simultaneously.

CONSTITUTION: Information Ii1 from a master unit is coded to a digital code by digital code converter 16 and is constituted by frames together with frame synchronizing pattern code and becomes a differential phase-shift keying wave (DPSK wave) and is transmitted from coil L1 of coupler 18 to the slave unit side. Coil L1 is resonated to the f wave by capacitor C1. The f wave subjected to DPSK is induced by coil L3, and the output is inputted in parallel to modulator 34, frequency 2-multiplier 24, DPSK demodulator 25 and square wave converter 28. Frequency 2- multiplier 24 outputs a continuous-phase 2f wave to bit synchronizing circuit 26. The output of DPSK demodulator 25 is transmitted from decoder 27 to the slave unit side as information output I01. Input information Ii2 of the slave unit side is coded to a digital code by the timing control of the master unit.