Transparent PEFC (Polymer Electrolyte Fuel Cell) to visualize and to analyze microscopic image of product water in gas channel and GDL (Gas Diffusion Layer) has been developed by three times trial and error design improvement. To obtain electrical contact and gas seal, MEA (Membrane Electrode Assembly), a pair of GDLs, polar plates, glass windows and gaskets are independently clamped by each bolts and nuts, which are effective to avoid performance decrease with running time and periodical change of output voltage caused by gas channel plugging. Cutting slits around gas channel in polar plates make out-plane rigidity lower and electrical contact resistance between GDL and MEA lower. This transparent PEFC enables us to observe micro scale product water exhaust from porous GDL, which could be effective to improve gas permeability and product water exhaustibility of GDL.