A thermoelectric semiconductor has a P-type semiconductor and an N-type semiconductor disposed in parallel. A heat absorbing side of the thermoelectric semiconductor and a substrate that has an optical element mounted on its upper surface are disposed on the same plane. A heat radiation side of the thermoelectric semiconductor is disposed such that a direction from the heat absorbing side to the heat radiation side of the thermoelectric semiconductor is parallel with the upper surface of the substrate. Based on this arrangement, it is possible to set the environmental temperature of an optical module to the same level as the operation temperature of a laser diode.