A selective catalytic reduction (SCR) system that can prevent fixation of urea solution to an interior of a dosing valve when the urea solution is not injected through the dosing valve. An SCR system closes a dosing valve when the exhaust gas temperature is lower than a temperature at which an SCR catalyst becomes active and is provided with a fixation prevention system that opens the dosing valve and controls a supply module to feed the urea solution in the dosing valve back to a urea tank to prevent fixation of the urea solution to the interior of the dosing valve, when the exhaust gas temperature is equal to or higher than a temperature at which the urea solution crystallizes and lower than the temperature at which the SCR catalyst becomes active.