It was tried to fabricate a WO 3 thin film device through a wet process starting from an aqueous sol of WO 3 .2H 2 O. When mixed with polyethylene glycol (PEG, molecular weight 6000), the sol was made compatible to spin coating on an alumina substrate and the coating could be converted into a WO 3 thin film by calcination at 300 o C for 2h. Starting with a typical coating dispersion containing WO 3 .2H 2 O by 5mass% on WO 3 basis and 2mass% PEG, the WO 3 film obtained was 450nm in mean thickness. The film was a slim pack of square plates, each of which was a stack of thin lamellar crystals of WO 3 . The device was sensitive enough to detect 50ppb NO 2 in air at 200 or 250 o C, although the response and recovery transients were rather sluggish. Unexpectedly, the transients were found to be sharpened drastically in humid atmosphere, while sensor response (sensitivity) to NO 2 was hardly degraded with humidity.