A Mach-Zehnder optical modulator capable of feed-back compensating a change due to an optical coupling loss, and the characteristic change peculiar to functional devices, and a control method therefore, are provided. A control method for a semiconductor Mach-Zehnder optical modulator using a laser device as a light source includes modulating light emitted from the laser device, and extracting the light as output light; detecting monitor light separate from the output light, among the optically modulated light; and feed-back controlling an optical output intensity of the laser device based on the monitor light.