An optical transmitter module is described. The optical transmitter module includes a lead pin for electrically connecting the inside and outside of a housing, and a flexible printed circuit board connected to the lead pin. The flexible printed circuit board has a signal pattern and two ground conductor patterns to be connected to an optical modulation element, a laser terminal pattern to be connected to a semiconductor laser, a Peltier terminal pattern to be connected to a Peltier element, and two covering conductive layers in addition to a layer on which such patterns are formed. The covering conductive layers cover all the patterns except for the signal pattern.