A dynamic range of intensity modulation is set to range from a maximum intensity Smax to a minimum intensity Smin. A difference Δ(=Smax−Smin) between the maximum intensity Smax and the minimum intensity Smin is divided by the number 2M of multilevel signals. Thus, a distance (an intensity difference) between adjacent signals is [Δ/2M]. The number 2M of multilevel signals is selected such that the distance [Δ/2M] between adjacent multilevel signals (between an intensity Si and an intensity Si+1) is sufficiently buried within a range of quantum fluctuations obtained when heterodyne measurements are made or buried within a range of quantum shot noise obtained when a direct detection is made. Bases of a basis group are each positioned for intensity signals so as to have a high intensity and a low intensity between which a distance is set to be a certain value smaller than a middle point intensity [Δ/2]. Note that adjacent bases are set to have an intensity signal transmitting a logical value “1” of transmission data and an intensity signal transmitting a logical value “0” of transmission data in an inverted manner.