The FM stereo broadcasting apparatus has a sampling frequency converter for receiving a digital input signal having an L (left audio)-signal and an R (right audio)-signal and for converting the digital input signal so that its sampling frequency is equal to either a frequency of a subcarrier of a composite audio signal or a first sampling frequency of a first sampling signal for the operation of a composite audio signal generator, a sampling signal generator for generating the first sampling signal for the operation of the composite audio signal generator having the first sampling frequency which is equal to 2.sup.N times (N is an integer of 1 or above) as high as the frequency of the subcarrier, a composite audio signal generator which is operated by the first sampling signal and which generates the composite audio signal on the basis of a pilot signal and the L-signal and R-signal of the input signal from the first frequency converter, and a frequency modulator for modulating the frequency of a carrier signal by the composite audio signal from the composite audio signal generator so as to produce a frequency-modulated signal.