An SCR system including: a selective catalytic reduction device provided in an exhaust pipe for an engine; a urea tank that stores urea; a urea level sensor provided in the urea tank, the urea level sensor including detection means for detecting a liquid level of the urea stored in the urea tank; and a urea injection control unit that controls an injected quantity of urea according to a quantity of NO x emitted from the engine, wherein the SCR system comprises: a urea injection quantity integrating unit that integrates the injected quantity of urea to calculate a urea injection quantity integrated value; an integrated value resetting unit that is configured to reset the urea injection quantity integrated value if a detected value from the detection means changes; and an abnormality determination unit that is configured to determine the urea level sensor as having an abnormality if the urea injection quantity integrated value exceeds a predetermined threshold value for determination, characterized in that the urea level sensor is a float-type urea level sensor including a float following a liquid surface of the urea in the tank, wherein the detection means in the urea level sensor is configured to detect a position of the float to detect the liquid level of the urea stored in the urea tank in a stepwise manner; and wherein the threshold value for determination is set to a value exceeding a quantity of urea corresponding to at least one step in steps of the liquid level of the urea detected by the detection means. a selective catalytic reduction device provided in an exhaust pipe for an engine; a urea tank that stores urea; a urea level sensor provided in the urea tank, the urea level sensor including detection means for detecting a liquid level of the urea stored in the urea tank; and a urea injection control unit that controls an injected quantity of urea according to a quantity of NO x emitted from the engine, wherein the SCR system comprises: a urea injection quantity integrating unit that integrates the injected quantity of urea to calculate a urea injection quantity integrated value; an integrated value resetting unit that is configured to reset the urea injection quantity integrated value if a detected value from the detection means changes; and an abnormality determination unit that is configured to determine the urea level sensor as having an abnormality if the urea injection quantity integrated value exceeds a predetermined threshold value for determination, a urea injection quantity integrating unit that integrates the injected quantity of urea to calculate a urea injection quantity integrated value; an integrated value resetting unit that is configured to reset the urea injection quantity integrated value if a detected value from the detection means changes; and an abnormality determination unit that is configured to determine the urea level sensor as having an abnormality if the urea injection quantity integrated value exceeds a predetermined threshold value for determination, characterized in that the urea level sensor is a float-type urea level sensor including a float following a liquid surface of the urea in the tank, wherein the detection means in the urea level sensor is configured to detect a position of the float to detect the liquid level of the urea stored in the urea tank in a stepwise manner; and wherein the threshold value for determination is set to a value exceeding a quantity of urea corresponding to at least one step in steps of the liquid level of the urea detected by the detection means.