A control method for a laser diode is conducted such that optical output and OMA (optical modulation amplitude) or extinction ratio of the laser diode are controlled while modulating the optical output of the laser diode by applying a bias current and a modulation current to the laser diode. The method has: a first step of measuring the optical output of the laser diode; a second step of measuring a slope efficiency or a corresponding value of the slope efficiency while conducting APC (automatic power control) to adjust the bias current such that the optical output coincides with a predetermined value; and a third step of adjusting the modulation current by AAC (automatic amplitude control) according to the slope efficiency or the corresponding value of the slope efficiency such that the OMA or extinction ratio coincides with a predetermined value.