Abstract: The present invention provides a universal serial bus function evaluator connected between a computer and an universal serial bus function. The universal serial bus function evaluator comprises: a token storage memory for storing a token transmitted from the computer; a packet type judging circuit for judging a type of a return data packet returned from the universal serial bus function; and a functional circuit connected to the token storage memory for fetching IN token from the token storage memory and holding the same, and the functional circuit also being connected to the packet type judging circuit for receiving an information about the type of the return data packet from the packet type judging circuit, so that if the return data packet is of NAK type, then the functional circuit transmits the IN token held therein to the universal serial bus function, and if the return data packet is of either DAT type or STALL type, then the functional circuit cancels the held IN token.