On a second thermo-module there are disposed a prism, an optical filter, and a sub-mount, via a base respectively. On the front side of the sub-mount, there are disposed a first photodetector and a second photodetector. The optical filter is structured with a crystal member that satisfies a condition that α+(1/n0)×(Δn/ΔT) becomes not more than 3.5, where α is a coefficient of linear expansion, n0 is a refractive index at a normal temperature, and Δn is a change of a refractive index corresponding to a temperature variation ΔT.