A 120-GHz-band close-proximity wireless system that achieves the first 10-Gbit/s real-time transmission satisfying the regulation for extremely low-power radio stations (ELRS regulation). The use of photonic technologies enables the transmitter to perform 10-Gbit/s quadrature-phase-shift keying (QPSK) modulation and generate an RF spectrum with negligibly small spurious signals. The phase of the emitted signal fluctuates with changes in the external environment, which interferes with the demodulation using a reference signal. To overcome phase fluctuation at the transmitter, we use differentially coherent detection for the demodulation. We verified that the level of the measured RF spectrum of the integrated transmitter was below the level defined by the ELRS regulation. The transmitter and the receiver demonstrated error-free wireless transmission over a distance of 10mm with a link margin of 11dB.