An SCR system including: an SCR (selective catalytic reduction) device provided in an exhaust pipe for an engine; a dosing valve that injects urea solution on an upstream side of the SCR device; a urea solution injection control unit that controls an injected quantity of urea solution according to a quantity of NO x emitted from the engine; and a minimum injection quantity setting unit that if a temperature of the dosing valve is equal to or higher than a predetermined temperature, sets a minimum injection quantity according to the temperature of the dosing valve, wherein the urea solution injection control unit is configured to, if the injected quantity of urea solution according to the quantity of NO x emitted from the engine is smaller than the minimum injection quantity set by the minimum injection quantity setting unit, control the injected quantity of urea solution to be the minimum injection quantity, the minimum injection quantity setting unit includes a load state determination unit that determines whether an operation state of the engine is a high-load operation state or a low-load operation state, and the minimum injection quantity setting unit is configured to set a high-load minimum injection quantity if the load state determination unit determines that the operation state of the engine is a high-load operation state, and set a low-load minimum injection quantity if the load state determination unit determines that the operation state of the engine is a low-load operation state, and the low-load minimum injection quantity is set to a value smaller than the high-load minimum injection quantity, characterized in that the SCR system further comprises a minimum injection quantity setting condition determination unit that permits the setting of the minimum injection quantity by the minimum injection quantity setting unit only if a predetermined minimum injection quantity setting condition is met, and the predetermined minimum injection quantity setting condition is met when the injected quantity of urea solution is reduced to be smaller than the injection quantity according to the quantity of NO x emitted from the engine for ammonia slip suppression. an SCR (selective catalytic reduction) device provided in an exhaust pipe for an engine; a dosing valve that injects urea solution on an upstream side of the SCR device; a urea solution injection control unit that controls an injected quantity of urea solution according to a quantity of NO x emitted from the engine; and a minimum injection quantity setting unit that if a temperature of the dosing valve is equal to or higher than a predetermined temperature, sets a minimum injection quantity according to the temperature of the dosing valve, wherein the urea solution injection control unit is configured to, if the injected quantity of urea solution according to the quantity of NO x emitted from the engine is smaller than the minimum injection quantity set by the minimum injection quantity setting unit, control the injected quantity of urea solution to be the minimum injection quantity, the minimum injection quantity setting unit includes a load state determination unit that determines whether an operation state of the engine is a high-load operation state or a low-load operation state, and the minimum injection quantity setting unit is configured to set a high-load minimum injection quantity if the load state determination unit determines that the operation state of the engine is a high-load operation state, and set a low-load minimum injection quantity if the load state determination unit determines that the operation state of the engine is a low-load operation state, and the low-load minimum injection quantity is set to a value smaller than the high-load minimum injection quantity, characterized in that the SCR system further comprises a minimum injection quantity setting condition determination unit that permits the setting of the minimum injection quantity by the minimum injection quantity setting unit only if a predetermined minimum injection quantity setting condition is met, and the predetermined minimum injection quantity setting condition is met when the injected quantity of urea solution is reduced to be smaller than the injection quantity according to the quantity of NO x emitted from the engine for ammonia slip suppression.