The present invention provides a compact, highly reliable optical module in which a light-emitting element and optical modulator element that differ from each other in thermal expansion coefficient are mounted in one casing. In the optical module having a differential thermal expansion coefficient less than 510^-6 [1/K] between the light-emitting element and the casing, and a differential thermal expansion coefficient of at least 510^-6 [1/K] between the optical modulator element and the casing, a section at which the optical modulator element and the casing are fixed is equal to or less than 10 mm wide in the direction of an optical axis.