Even if a size of a read image is large, a code search process can be speeded up to enable immediate output of a decoding result. Each time a predetermined number of lines of image data is taken, a pre-processing circuit executes pre-processing on the image data and calculates a characteristic amount indicating the likelihood of the code for each area in the image data. A processor determines a code candidate area based on the acquired characteristic amount and executes a decoding process of the determined area to generate a decoding result.