An apparatus and method for transferring hidden signals in a boundary scan test interface is disclosed, which defines an invalid state transition loop in a boundary scan test interface and initially monitors an input of state transition diagram of the boundary scan test interface so that an output of a first data is generated when a first predetermined input stream conforming to the invalid state transition loop is detected, and then an output of a second data is generated when a second predetermined input stream is detected, wherein the second predetermined input stream is different from the first one and also conforms to the invalid state transition loop.