A two-way transmission system of the present invention is arranged in such
a manner that, when a host-side controller confirms the receipt of an IN
packet from a USB host, the host-side controller transfers the IN packet
to a function-side controller, and receives a DATA packet from the
function side. The received DATA packet is temporarily stored in a FIFO.
At the time of the receipt of the IN packet again, the host-side
controller supplies, to the USB host, the DATA packet stored in the FIFO.