1. A USB host system for performing USB data transfer with a USB device connected thereto, the system comprising: a USB host controller which is compliant with an OHCI standard and includes a transfer memory for USB data transfer; a memory; a data transfer processing block for performing internal data transfer between the memory and the transfer memory; and transfer descriptor setting means for setting, for the USB host controller, a transfer descriptor for executing USB data transfer, wherein in the transfer memory, first through Nth (N is an integer of 2 or larger) transfer descriptor regions are allocated, and wherein the transfer descriptor setting means is configured so as to be able to set, for one end point of the USB device, a plurality of transfer descriptors using the first through Nth transfer descriptor regions, respectively, for the USB host controller, wherein when data is read from the USB device, the USB host controller transfers data read out from the USB device to one of the first through Nth transfer descriptor regions according to a mth (m is an integer of 1 or more) transfer descriptor, and continuously transfers data read out from the USB device to another of the first through Nth transfer descriptor regions according to a (m+1)th transfer descriptor, and after notified of completion of the data transfer according to the mth transfer descriptor from the USB host controller, the transfer descriptor setting means controls the data transfer processing block to execute internal data transfer processing of transferring data stored in the one of the first through Nth transfer descriptor regions to the memory in parallel to the data transfer by the USB host controller according to the (m+1)th transfer descriptor.