An AM data multiplexing modulation apparatus for multiplexing a digital signal to an AM modulation signal without adversely affecting an AM sync detection output even if AM sync detection is performed. An AM modulator AM modulates a carrier having a frequency fc by an input signal, an orthogonal modulator 3 orthogonally modulates a carrier having a frequency (fc+f.alpha.) by a QPSK baseband digital signal generated by a QPSK baseband digital signal generator 2, a sign inverter and a complex conjugate unit change the sign of a signal having the same phase as a reference carrier of vector shift and corresponding to a dibit formed by the QPSK baseband digital signal generated by the QPSK baseband digital signal generator, another orthogonal modulator orthogonally modulates a carrier having a frequency (fc-f.alpha.) by an output of the complex conjugate unit, the outputs of the two orthogonal modulators are added by an adder, this addition result is added to the output of the AM modulator by another adder which outputs an AM data multiplexing modulation output.