An image scanning device includes a scanning unit configured to execute image scanning of a document and thereby generate image data, at least one processing unit configured to successively process the image data outputted from the scanning unit, a transmission unit configured to transmit the image data outputted from the processing unit, a storage unit, in which multiple buffer areas to be used for transferring the image data among the scanning unit, the at least one processing unit and the transmission unit are allocated, an acquisition unit configured to acquire information on usage status of each of the buffer areas, and a changing unit configured to change storage area allocation at least between two of the buffer areas based on the usage status acquired by the acquisition unit.