This invention relates to a reducing agent supply device and an exhaust gas denitrification system using the same. The reducing agent supply device includes a reducing agent supply unit for supplying a reducing agent to a selective supply unit, a water supply unit for supplying water to the selective supply unit, the selective supply unit for selectively supplying any one of the reducing agent and water supplied from the reducing agent supply unit and the water supply unit to a spray unit, and the spray unit for spraying the reducing agent or water supplied from the selective supply unit, wherein the selective supply unit supplies water to the spray unit when the spray unit is blocked due to solidification of the reducing agent which remains behind therein because of a process in which the spray unit continuously sprays the reducing agent or a temporary malfunction of the device, thereby preventing blockage of the spray unit. Also an exhaust gas denitrification system using the device is provided.