In order to simultaneously realize high-speed data transmission, low-cost and lightweight network in automotive applications, a 3(lambda) WDM has been considered to be a suitable solution. We have developed two types of low-cost WDM modules using a polymer optical circuit. As the first method, we developed a planar optical circuit for interconnection between an optical fiber and an LD/PD chip using a PMMA waveguide having out-of-plane branching mirrors, fabricated simultaneously with a waveguide from a polymer film, using a newly-developed temperature-controlled RIE (Reactive Ion Etching) technique. The waveguide has a large diameter (100 x 100micrometers ) and a high-(Delta) (NA=0.5) structure. Transmission losses were 0.1 dB/cm (660nm), and 0.3 dB/cm (1320nm). As the second method, we employed a light-induced waveguide technology for the WDM module fabrication. When a GI optical fiber is soaked in two kinds of light-hardening resin blends in which the hardening wavelength differs ((lambda) 1>(lambda) 2) , the resin is irradiated with (lambda) 1 light through the fiber, a hardened polymer waveguide having uniform diameter geometry would grow from the fiber tip. This method does not need any fiber alignment and packaging process; therefore, cost-effective module fabrication can be expected. This technique utilizes the self-focusing phenomenon of the optical fiber's outgoing beam by a refractive index increase in the material hardening process. We could also confirm two key component formations necessary for the WDM module: regrowth of the waveguide form the backside of a WDM filter after passing through it, and a 90 degree(s) reflected waveguide using a 45 degree(s) mirror.