When in an optical signal amplifying triode 10, light of a second wavelength .lamda..sub.2, selected from among light from a first optical amplifier 26, into which a first input light L.sub.1 of a first wavelength .lamda..sub.1 and a second input light L.sub.2 of second wavelength .lamda..sub.2 have been input, and a third input light (control light) L.sub.3of a third wavelength .lamda..sub.3 are input into a second optical amplifier 34, an output light L.sub.4 of the third wavelength .lamda..sub.3, selected from among the light output from the second optical amplifier 34, is light that is modulated in response to the intensity variation of one or both of the first input light L.sub.1 of the first wavelength .lamda..sub.1 and the third input light L.sub.3 of the third wavelength .lamda..sub.3 and is an amplified signal, with which the signal gain with respect to the third input light (control light) L.sub.3 of the third wavelength .lamda..sub.3 is of a magnitude of 2 or more. An optical signal amplifying triode 10, which can directly perform an optical signal amplification process using control input light, can thus be provided.