To solve a major problem of facilitate start-up of nodes of an optical transmitter to which closed loop topology is applicable.

The optical transmitter of this invention is provided with a closed loop topology using an optical fiber 2 applicable to a WDM signal light obtained by applying wavelength division multiplex processing to a plurality of optical signals, and a plurality of nodes 4 each including an optical amplifier provided along the closed loop topology. Each node includes a first means for controlling the optical amplifier so that the optical amplifier provides a constant output in response to the number of channels for the WDM signal light sent from the immediately upstream node of the node concerned , and a second means for shielding the optical signal so as to prevent the node concerned from outputting optical signal other than a single or a plurality of optical signals applied to the node concerned.