An exhaust gas purification apparatus of an engine comprising: a nitrogen oxide reduction catalyst arranged in an engine exhaust gas passage to reduce and purify nitrogen oxide in exhaust gas using a liquid reducing agent; a nozzle having an injection hole that opens into the exhaust gas passage, and positioned on an exhaust gas upstream side of said nitrogen oxide reduction catalyst; an operating state detecting device that detects an engine operating state; a reducing agent injection-supply device capable of injection-supplying a liquid reducing agent into the exhaust gas passage from said nozzle injection hole, based on the engine operating state detected by said operating state detecting device; and a control unit with a built-in computer; a solenoid-operated openably closed valve between a high pressure air supply device and said reducing agent injection supply device; and characterised by comprising a pressure-reducing device between said openably closed valve and said reducing agent injection supply device, and switchable to either let compressed air pass therethrough directly, or reduce the pressure of compressed air as it passes therethrough; wherein the control unit is configured to open the openably closed valve and control the pressure-reducing device to reduce the pressure of compressed air passing therethrough when the liquid reducing agent is supplied; and the control unit is configured to open the openably closed valve for a predetermined period after injection of the liquid reducing agent is stopped and control the pressure-reducing device to let compressed air pass therethrough directly. a nitrogen oxide reduction catalyst arranged in an engine exhaust gas passage to reduce and purify nitrogen oxide in exhaust gas using a liquid reducing agent; a nozzle having an injection hole that opens into the exhaust gas passage, and positioned on an exhaust gas upstream side of said nitrogen oxide reduction catalyst; an operating state detecting device that detects an engine operating state; a reducing agent injection-supply device capable of injection-supplying a liquid reducing agent into the exhaust gas passage from said nozzle injection hole, based on the engine operating state detected by said operating state detecting device; and a reducing agent injection-supply device capable of injection-supplying a liquid reducing agent into the exhaust gas passage from said nozzle injection hole, based on the engine operating state detected by said operating state detecting device; and a control unit with a built-in computer; a solenoid-operated openably closed valve between a high pressure air supply device and said reducing agent injection supply device; and characterised by comprising a pressure-reducing device between said openably closed valve and said reducing agent injection supply device, and switchable to either let compressed air pass therethrough directly, or reduce the pressure of compressed air as it passes therethrough; wherein the control unit is configured to open the openably closed valve and control the pressure-reducing device to reduce the pressure of compressed air passing therethrough when the liquid reducing agent is supplied; and the control unit is configured to open the openably closed valve for a predetermined period after injection of the liquid reducing agent is stopped and control the pressure-reducing device to let compressed air pass therethrough directly.