An optical module includes an optical element mount substrate having a laser diode and a photo detector mounted thereon on a straight line in a horizontal direction of the surface of the substrate, a first optical function integrate substrates, provided in its one surface with surfaces tilted at an angle to the parallel surface of the substrate for refracting light propagated in a substrate thickness direction, the two tilted surfaces being arranged to face each other. The photo detector and the first and second optical function integrate substrates are laminated in the substrate thickness direction, a first optical wavelength filter and a reflecting film having different wavelength transmissivities to the propagation light are provided in an optical path between the first and second tilted surfaces.