An urea solution injection device of an exhaust purification device in which an urea solution is added as a reducing agent to exhaust gas of an internal combustion engine so as to reduce nitrogen oxide in the exhaust gas, comprising: a temperature sensor adapted to detect a temperature of atmosphere; a humidity sensor adapted to detect an absolute humidity or a relative humidity of the atmosphere; and a control device adapted to calculate an addition amount of the urea solution, characterized in that the control device further comprises a map, which is adapted to convert an actual NOx discharge amount (Nr) of the internal combustion engine driven with each rotation speed and each load under predetermined temperature and absolute humidity of the atmosphere into the standard NOx discharge amount (Ns) of the internal combustion engine with each rotation speed and each load under standard conditions with a correction formula is stored in the control device, a rotation speed detection means adapted to detect the rotation speed of the internal combustion engine and a load detection means adapted to detect the load of the internal combustion engine are connected to the control device, and wherein the control device is adapted such that the standard NOx discharge amount (Ns) corresponding to the rotation speed detected by the rotation speed detection means and the load detected by the load detection means is calculated with the map, the standard NOx discharge amount (Ns) is converted into the actual NOx discharge amount (Nr) of the rotation speed and the load under the temperature of the atmosphere detected by the temperature sensor and the absolute humidity of the atmosphere detected by the humidity sensor with the correction formula by inverse operation, and the addition amount of the urea solution is calculated based on the actual NOx discharge amount (Nr). a temperature sensor adapted to detect a temperature of atmosphere; a humidity sensor adapted to detect an absolute humidity or a relative humidity of the atmosphere; and a control device adapted to calculate an addition amount of the urea solution, characterized in that the control device further comprises a map, which is adapted to convert an actual NOx discharge amount (Nr) of the internal combustion engine driven with each rotation speed and each load under predetermined temperature and absolute humidity of the atmosphere into the standard NOx discharge amount (Ns) of the internal combustion engine with each rotation speed and each load under standard conditions with a correction formula is stored in the control device, a rotation speed detection means adapted to detect the rotation speed of the internal combustion engine and a load detection means adapted to detect the load of the internal combustion engine are connected to the control device, and wherein the control device is adapted such that the standard NOx discharge amount (Ns) corresponding to the rotation speed detected by the rotation speed detection means and the load detected by the load detection means is calculated with the map, the standard NOx discharge amount (Ns) is converted into the actual NOx discharge amount (Nr) of the rotation speed and the load under the temperature of the atmosphere detected by the temperature sensor and the absolute humidity of the atmosphere detected by the humidity sensor with the correction formula by inverse operation, and the addition amount of the urea solution is calculated based on the actual NOx discharge amount (Nr).