A sufficient decoding processing time for each image acquired by performing high-speed imaging is secured to obtain a stable reading result, thereby enabling immediate output of the obtained reading result. A processing unit has a first core and a plurality of second cores. The first core instructs the second cores, presumed to be capable of immediately executing the decoding process or executing the decoding process next to a decoding process being currently executed, to execute the decoding process. The second cores are configured to be capable of simultaneously executing the decoding process on read images instructed by the first core at different timings.