We have fabricated the catalytic tubular reactor whose inner wall is coated with thin palladium oxide (PdO) layer (ca. 3 mu m). Continuous decomposition of aqueous formic acid (HCO2H) was examined by the flow reaction system installed with the catalytic tubular reactor. Reactor with PdO inner surface showed superior catalytic performance than that obtained by the reactor having metallic Pd surface. Higher temperature and longer residence time increased the reaction conversion but the effect of pressure was small. Steady generation of hydrogen (H-2) was achieved keeping >99% conversion of formic acid at 300 degrees C (10 MPa) within 4.3 s residence time. H-2 and carbon dioxide (CO2) were major products with ca. 1 : 1 molar ratio and the generation of carbon monoxide (CO) was minor. Generation of CO was considerably suppressed by increase of sodium formate ratio i.e., formate/(formate+formic acid) ratio. Long term test revealed that steady productivity of H-2 was maintained more than 600 h reaction. The XPS spectrum of catalytic surface after long time use evidenced the presence of PdO in large excess along with small amount of metallic Pd indicating that reduction of PdO surface proceeded quite slowly.