Reducing agent injection device and exhaust gas treatment method

A reducing agent injection device, comprising:

• a first honeycomb structure that has a pillar-shaped honeycomb structure body having partition walls defining and forming a plurality of cells which is through channels of a fluid and extends from a first end face being an end face on an inflow side of the fluid to a second end face being an end face on an outflow side of the fluid, and that has at least a pair of electrode members arranged in a side surface of the honeycomb structure body;
• and a urea spraying device that sprays a urea water solution in mist form, wherein each of the pair of electrode members is formed in a band shape extending to a cell extending direction of the honeycomb structure body, and one electrode member of the pair of electrode members is arranged on an opposite side of the other electrode member of the pair of electrode members with respect to a center of the honeycomb structure body sandwiched by the pair of electrode members, in a cross section perpendicular to the cell extending direction, 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, and the urea water solution sprayed from the urea spraying device is supplied into the cells from the first end face of the honeycomb structure body, and urea in the urea water solution supplied into the cells is heated and hydrolyzed in the electrically heated honeycomb structure body to generate ammonia, and the ammonia is discharged outside the honeycomb structure body from the second end face and is injected outside.