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; an NO x sensor provided in the exhaust pipe on the upstream side of the SCR device; and a urea solution injection control unit that controls a urea solution injection quantity according to an NO x concentration value detected by the NO x sensor, a pressure estimation unit that estimates an NO x sensor pressure that is a pressure inside the exhaust pipe at a position where the NO x sensor is provided, and an NO x concentration detection value correction unit that corrects the NO x concentration value detected by the NO x sensor, according to the NO x sensor pressure estimated by the pressure estimation unit, wherein the pressure estimation unit comprises an exhaust gas flow estimation unit that estimates an exhaust gas flow by summing an intake air flow of the engine and a fuel flow calculated from an engine rotation speed and a fuel injection quantity, an exhaust gas specific volume estimation unit that estimates an exhaust gas specific volume, based on an SCR entrance temperature that is an exhaust gas temperature at an entrance of the SCR device, and a barometric pressure, an SCR differential pressure estimation unit that estimates a differential pressure between a pressure at the entrance of the SCR device and the barometric pressure, based on the exhaust gas flow estimated by the exhaust gas flow estimation unit, the exhaust gas specific volume estimated by the exhaust gas specific volume estimation unit, and the SCR entrance temperature, and an NO x sensor pressure estimation unit that estimates an NO x sensor pressure by adding the barometric pressure to the differential pressure estimated by the SCR differential pressure estimation unit. 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; an NO x sensor provided in the exhaust pipe on the upstream side of the SCR device; and a urea solution injection control unit that controls a urea solution injection quantity according to an NO x concentration value detected by the NO x sensor, a pressure estimation unit that estimates an NO x sensor pressure that is a pressure inside the exhaust pipe at a position where the NO x sensor is provided, and a pressure estimation unit that estimates an NO x sensor pressure that is a pressure inside the exhaust pipe at a position where the NO x sensor is provided, and an NO x concentration detection value correction unit that corrects the NO x concentration value detected by the NO x sensor, according to the NO x sensor pressure estimated by the pressure estimation unit, wherein the pressure estimation unit comprises an exhaust gas flow estimation unit that estimates an exhaust gas flow by summing an intake air flow of the engine and a fuel flow calculated from an engine rotation speed and a fuel injection quantity, an exhaust gas specific volume estimation unit that estimates an exhaust gas specific volume, based on an SCR entrance temperature that is an exhaust gas temperature at an entrance of the SCR device, and a barometric pressure, an SCR differential pressure estimation unit that estimates a differential pressure between a pressure at the entrance of the SCR device and the barometric pressure, based on the exhaust gas flow estimated by the exhaust gas flow estimation unit, the exhaust gas specific volume estimated by the exhaust gas specific volume estimation unit, and the SCR entrance temperature, and an NO x sensor pressure estimation unit that estimates an NO x sensor pressure by adding the barometric pressure to the differential pressure estimated by the SCR differential pressure estimation unit.