An optical transmitter module includes a semiconductor laser for outputting forward outgoing light and backward outgoing light, a temperature control device for controlling a temperature of the semiconductor laser, a beam splitter plate for receiving incidence of the backward outgoing light and outputting split light, which is reflected part of the backward outgoing light and transmitted light, which is part of the backward outgoing light, a first photoelectric conversion element for converting the split light into an electric signal. The beam splitter plate includes an anti-stray-light structure for preventing the transmitted light reflected by an incident surface of a wavelength filter from entering the first photoelectric conversion element through a side end surface portion of the beam splitter plate.