reference to fig5 a to 5f of the relationship between the temperature of the semiconductor optical amplifier 30 and the gain thereof in a case where preheat current is supplied to the active layer of the semiconductor optical amplifier 30 in a case where no optical signal is input to the semiconductor optical amplifier 30 as illustrated in fig5 b predetermined preheat current is supplied to the active layer of the semiconductor optical amplifier 30 as illustrated in fig5 a in this case in the semiconductor optical amplifier 30 heat is generated resulting from the preheat current as a result a given temperature slope is formed in the semiconductor optical amplifier 30 and the temperature of the active layer of the semiconductor optical amplifier 30 becomes higher than the temperature of the element mounted surface of the thermo electric cooler 20 next in a case where an optical signal is input as illustrated in fig5 d driving current is supplied to the active layer of the semiconductor optical amplifier 30 in accordance with the correspondence table 71 as a result in the semiconductor optical amplifier 30 the optical signal is amplified in this case since a temperature slope has already been formed in the semiconductor optical amplifier 30 the temperature change in the.