The optical amplifying device comprises a DFB laser 22 formed on an n type InP substrate 10, for outputting control light; a symmetrical Mach-Zehnder interferometer 12 formed on the n type InP substrate 10 and including 3 dB optical couplers 14, 16 having 2 input ports and 2 output ports, and optical waveguides 24a, 24b which optically interconnect the output port of the 3 dB optical coupler 14 and the input port of the 3 dB optical coupler 16; and SOAs 24a, 24b respectively formed in the optical waveguides 24a, 24b.