An optical modulating circuit realizing a PPM is constituted by circuit parts having a frequency band equal to a data rate to provide a technique which increase the data rate. A first light source generates a first single wavelength signal serving as a continuous light having a first wavelength .lamda.1 as a wavelength of a carrier wave. A second light source generates a second single wavelength signal serving as a continuous light having a second wavelength .lamda.2 different from .lamda.1 as a wavelength of a carrier wave. For the first single wavelength signal and the second single wavelength signal, according to a transmission electric signal having information 0 or 1, an optical switch outputs a first single wavelength signal as an input optical signal when the information is 0. On the other hand, when the information is 1, the optical switch outputs the second single wavelength signal as an input optical signal. An optical modulator generates an optical pulse signal obtained by arranging optical pulses having .lamda.1 or .lamda.2 as a wavelength of a carrier wave at predetermined time intervals from the input optical signal. A delaying unit delays the optical pulse having the wavelength .lamda.1 and the optical pulse having the wavelength .lamda.2 by different delay time .DELTA.t to obtain a PPM signal.