Exhaust gas purifying apparatus and exhaust gas purifying method of an engine

An exhaust gas purifying apparatus for an engine, comprising:

• addition means for adding a reducing agent of NOx to an exhaust gas of the engine;
• storage means for storing the reducing agent of NOx that is added to the exhaust gas by the addition means or a precursor thereof in a state of an aqueous solution;
• concentration sensing means for sensing a concentration of the reducing agent or the precursor contained in the aqueous solution of the reducing agent or the precursor that is stored in the storage means;
• command generating means for generating an operation command to a predetermined object for being controlled related to purifying of the exhaust gas on the basis of the detected concentration which is the concentration of the reducing agent or the precursor detected by the concentration sensing means;
• state determining means for determining whether or not the aqueous solution of the reducing agent or the precursor is in a stationary state in the storage means;
• and
• sensing permission means for permitting sensing of the concentration by the concentration sensing means at a stationary time when the aqueous solution of the reducing agent or the precursor is determined to be in the stationary state by the state determining means, while prohibiting the sensing of the concentration by the concentration sensing means at a shaking time other than the stationary time, wherein
• the concentration sensing means detects the concentration of the reducing agent or the precursor only at the stationary time.