In this paper, to obtain higher spectral efficiency than a conventional M-ary/Spread Spectrum (SS) communication system using an orthogonal code, we propose Double M-ary/SS communication systems in which transmitting sequences are produced by multiplying two sequences obtained from different M-ary/SS systems. First, we estimate the system performance of a Double M-ary/SS in which transmitting sequences are composed of two kinds of sequences which have the same chip duration and the same sequence length in AWGN environment by theoretical analysis and computer simulations using random sequences. And we show that the bit error rate (BER) of the system can be improved, compared to that of the conventional M-ary/SS. Next, we propose two concrete examples of the Double M-ary/SS systems. We investigate the performance of these systems in AWGN environment by computer simulations, and show that the BER and spectral efficiency can be improved by the two systems compared to those of the conventional M-ary/SS. Further, we consider a modified method of the demodulation for one of the examples of Double M-ary/SS systems, and show that we can reduce quantity of calculations by the method.