This paper describes a novel method for fabricating paper-based microfluidic devices using a laser beam scanning technique. Cellulose chromatography papers were treated with octadecyltrichlorosilane (OTS) to make them entirely hydrophobic. A photoacid generator (CPI-410S) was soaked into the paper, and irradiated with a 405-nm laser beam to induce acid generating reactions. Since the silyl ether bond between cellulose and OTS was cleaved by the hydrolysis reaction, the photo-irradiated area changed to hydrophilic. By scanning the laser beam using a Galvo mirror system, arbitrary shaped hydrophilic patterns were successfully created on the paper in 50 μm resolution. To the best of our knowledge, this is the first report on the fabrication of hydrophilic channels on the OTS-treated paper using photo-induced acid generation processes coupled with the laser beam scanning technique. Quantification of nitrite was demonstrated with the paper device made by this method.