PURPOSE: To provide a rare earth doped optical fiber type optical amplifier which is extremely less fluctuated in amplification characteristics and noise characteristics with an environmental temp. change and is extremely high in a stability of these performance characteristics at the time of using the amplifier as an optical communication system.

CONSTITUTION: Heating and cooling means are joined to a supporting body 2 of the optical amplifier which amplifies signal light 20 with the induction released light obtd. by making the signal light 20 and exciting light 22 incident on an optical fiber 1 which is doped with a rare earth element and is wound around the supporting body 2. The material of the supporting body 2 is a thermal conductor consisting of metal. The heating and cooling means consist of a Peltier element 9, a temp. sensor 10, a comparator circuit 11 which compares the temp. measured by the temp. sensor 10 and a preset temp. and a working control circuit 12 which controls the operation of the Peltier element 9 by the result of comparison made by the comparator circuit 11. The height of winding of the optical fiber 1 wound on the supporting body 2 is above the winding width.