A signal adding device and a differential signal detecting device, wherein the signal adding device adds n input signals using n A/D converters, a selector and a lowpass filter. The n input signals are sampled in sequence by the A/D converter and digitized. Each digitized data is time-division multiplexed by the selector and input to the lowpass filter. The output of the lowpass filter is equal to an added signal of n input signals. The differential signal detecting device detects from inphase and quadrature-phase baseband signals the sine and cosine components of a phase difference at two instants of the phase-modulated signal. Two A/D converters digitize and convert the inphase and quadrature-phase baseband signals to inphase data and quadrature-phase data at predetermined intervals. A cosine component calculating unit calculates the cosine component by alternately selecting the inphase data and the quadrature-phase data. Similarly, a sine component calculating unit calculates the sine component by alternately selecting the inphase data and the quadrature-phase data. Two selectors select either the inphase data or the quadrature-phase data at predetermined intervals. By alternately selecting the inphase data and quadrature-phase data, and then multiplying the results, differential detection can be achieved with fewer multipliers and adders than the prior art differential detecting devices.