In the NH3 injection rate control method for an NOx removal apparatus, operating area of the apparatus is divided into first area in which molar ratio of NH3 injection rate with respect to NOx flow rate at an inlet of the apparatus is smaller than molar ratio at minimum point where NOx concentration assumes minimum value and second area in which the molar ratio is equal to or larger than the minimum point molar ratio, and virtual NOx concentration with respect to the molar ratio is set according to virtual characteristic line which varies monotonically to stride across desired operating point without rising with increase of the molar ratio from the first area to the second area. Moreover, feedback control is implemented for adjusting the NH3 injection rate with respect to the NOx flow rate in direction that the virtual NOx concentration is brought close to the desired NOx concentration.