An image processing apparatus, and a method of controlling the same, for making it possible to execute decoding of compressed image data appropriately, in a processing time needed for decoding that will not detract from user convenience, and in conformity with amount of code error contained in the image. An encoded data input signal has its header information analyzed by a header information processor, which outputs an encoding-parameter output signal. The image information of the encoded data input signal is decoded by a variable-length code decoder, which produces an output signal that is a series of orthogonal transformation coefficient sequences. These two output signals are input to a image data reconstruction unit, which reconstructs the image data. If the encoded data input signal contains a code error, then the encoded data input signal is corrected using correction data. The correction data for correcting the encoded data input signal is decided based upon the number of detection of code errors.