A reducing agent injection device includes a first honeycomb structure and a urea spraying device spraying a urea water solution in mist form. A pair of electrode members is formed in the first honeycomb structure. The ratio L/D of length L in the cell extending direction of the honeycomb structure body to diameter D of the cross section perpendicular to the cell extending direction is 0.5 to 1.2. Also, it is preferable that a urea hydrolysis catalyzer is provided in the second end face side of the honeycomb structure body, with a gap from the second end face.
