PURPOSE: To provide an analog modulation type transmission system proper to the transmission of a long span by low distortion by using a laser chip, in which a coupling coefficient is changed, and adjusting the degree of modulation regarding an analog modulator for a semiconductor laser enabling long- distance transmission.

CONSTITUTION: In an analog modulator for a semiconductor laser, in which a modulation signal is overlapped to the specified bias of a semiconductor laser diode 1 and analog modulation is conducted, a bias-current adjusting device 5, in which a laser chip, in which a gradient to bias currents of the differential efficiency of an optical output draws a quadratic curve, is used as a laser diode and which imparts a bias value at a point, where the gradient reaches zero, and an amplitude adjusting device 6 adjusting the degree of modulation of a modulation signal are provided.