An SCR system comprising: a 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 selective catalytic reduction device; an NO x sensor (110, 111) that detects an NO x concentration in an exhaust gas; a urea solution injection control unit that controls the urea solution injection according to the NO x concentration detected by the NO x sensor (110, 111); and an electric heater (2, 3) for raising a temperature of the NO x sensor (110, 111) to a sensor operating temperature; characterized by an energization permission unit that prohibits energization of the heater (2, 3) at the time of a start of the engine, and permits energization of the heater (2, 3) when engine revolutions become equal to or higher than a preset energization permission threshold value and such state lasts for a period of time equal to or exceeding a preset stabilization wait time, wherein the energization permission unit further maintains to permit energization of the heater (2, 3) so as to maintain the state in which the NO x concentration can be detected by the NO x sensor (110, 111) when the engine revolutions become equal to or fall below a preset energization prohibition threshold value during operation of the engine, prohibits energization of the heater (2, 3) when the state in which the engine revolutions become equal to or fall below the energization prohibition threshold value lasts for a period of time equal to or exceeding a preset recovery wait time, and continues energization of the heater (2, 3) when the engine revolutions become equal to or fall below the energization prohibition threshold value and the energization permission of the heater is maintained so as to maintain the state in which the NO x concentration can be detected by the NO x sensor (110, 111) and when the engine revolutions rise and exceed the energization prohibition threshold value before the state in which the engine revolutions become equal to or fall below the energization prohibition threshold value lasts for a period of time equal to or exceeding the recovery wait time. a 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 selective catalytic reduction device; an NO x sensor (110, 111) that detects an NO x concentration in an exhaust gas; a urea solution injection control unit that controls the urea solution injection according to the NO x concentration detected by the NO x sensor (110, 111); and an electric heater (2, 3) for raising a temperature of the NO x sensor (110, 111) to a sensor operating temperature; characterized by an energization permission unit that prohibits energization of the heater (2, 3) at the time of a start of the engine, and permits energization of the heater (2, 3) when engine revolutions become equal to or higher than a preset energization permission threshold value and such state lasts for a period of time equal to or exceeding a preset stabilization wait time, wherein the energization permission unit further maintains to permit energization of the heater (2, 3) so as to maintain the state in which the NO x concentration can be detected by the NO x sensor (110, 111) when the engine revolutions become equal to or fall below a preset energization prohibition threshold value during operation of the engine, prohibits energization of the heater (2, 3) when the state in which the engine revolutions become equal to or fall below the energization prohibition threshold value lasts for a period of time equal to or exceeding a preset recovery wait time, and continues energization of the heater (2, 3) when the engine revolutions become equal to or fall below the energization prohibition threshold value and the energization permission of the heater is maintained so as to maintain the state in which the NO x concentration can be detected by the NO x sensor (110, 111) and when the engine revolutions rise and exceed the energization prohibition threshold value before the state in which the engine revolutions become equal to or fall below the energization prohibition threshold value lasts for a period of time equal to or exceeding the recovery wait time.