An exhaust gas purification apparatus for an internal combustion engine comprising: an NOx selective reduction catalyst that is arranged in an exhaust passage of the internal combustion engine and selectively reduces NOx by the supply of a reducing agent; a tank that stores said reducing agent; a reducing agent passage that connects between the exhaust passage of said internal combustion engine and said tank; and an injection device that is opened when the reducing agent is supplied to said exhaust passage from said reducing agent passage, and is closed when the supply of the reducing agent is stopped; wherein a gaseous reducing agent is supplied to said exhaust passage from said reducing agent passage; said exhaust gas purification apparatus comprising: a pressure detection device that is arranged at a location from said tank to said injection device, and detects a pressure of the reducing agent; a temperature detection device that is arranged at a location from said tank to said injection device, and detects a temperature of the reducing agent; a determination device (10, S104) that determines whether the reducing agent leaks, according to a difference between a saturated vapor pressure corresponding to the temperature of the reducing agent detected by said temperature detection device at the time when said injection device is closed, and the pressure of the reducing agent detected by said pressure detection device at the time when said injection device is closed; and a correction unit (10, S202) that performs correction in such a manner that the smaller the amount of the reducing agent in said tank, the lower becomes said saturated vapor pressure. an NOx selective reduction catalyst that is arranged in an exhaust passage of the internal combustion engine and selectively reduces NOx by the supply of a reducing agent; a tank that stores said reducing agent; a reducing agent passage that connects between the exhaust passage of said internal combustion engine and said tank; and an injection device that is opened when the reducing agent is supplied to said exhaust passage from said reducing agent passage, and is closed when the supply of the reducing agent is stopped; wherein a gaseous reducing agent is supplied to said exhaust passage from said reducing agent passage; said exhaust gas purification apparatus comprising: a pressure detection device that is arranged at a location from said tank to said injection device, and detects a pressure of the reducing agent; a temperature detection device that is arranged at a location from said tank to said injection device, and detects a temperature of the reducing agent; a determination device (10, S104) that determines whether the reducing agent leaks, according to a difference between a saturated vapor pressure corresponding to the temperature of the reducing agent detected by said temperature detection device at the time when said injection device is closed, and the pressure of the reducing agent detected by said pressure detection device at the time when said injection device is closed; and a correction unit (10, S202) that performs correction in such a manner that the smaller the amount of the reducing agent in said tank, the lower becomes said saturated vapor pressure. a pressure detection device that is arranged at a location from said tank to said injection device, and detects a pressure of the reducing agent; a temperature detection device that is arranged at a location from said tank to said injection device, and detects a temperature of the reducing agent; a determination device (10, S104) that determines whether the reducing agent leaks, according to a difference between a saturated vapor pressure corresponding to the temperature of the reducing agent detected by said temperature detection device at the time when said injection device is closed, and the pressure of the reducing agent detected by said pressure detection device at the time when said injection device is closed; and a correction unit (10, S202) that performs correction in such a manner that the smaller the amount of the reducing agent in said tank, the lower becomes said saturated vapor pressure.