In nuclear industry, removal of nitric acid from solutions is required in the course of chemical separation and waste treatment procedure as well as in chemical conversion steps. The reduction of HNO3 by HCOOH to gaseous products such as nitrogen, nitrogen oxides, and carbon dioxide is an attractive way to accomplish the denitration. A typical problem for the denitration is the existence of the induction period. The induction period has been explained as the time necessary to increase the concentration of HNO2, which is an important reaction intermediate, above a threshold value. In this study, adsorption sites on the surface of active carbon were found to promote HNO2 formation and efficiently suppress the induction period. Induction time was shortened by increasing the amount of active carbon in the solution. When the solution contains 3 M HNO3 and 1 M HCOOH, 10 g/L of active carbon was enough to eliminate the induction period at 50 degrees C. The catalytic effect exhibited by active carbon was similar to that reported for Pt/SiO2. Therefore, on the surface of active carbon, there is a redox cycle, where HNO3 is reduced to HNO2 and then the oxidized surface site will be reduced by HCOOH.