1. A testing apparatus for an integrated circuit including a plurality of shift registers, to which test patterns are inputted, configured with sequential circuit elements, said testing apparatus comprising: a mask to specify a shift register in said plurality of shift registers which outputs an indeterminate value, based on external control signals, and to mask the indeterminate value by converting the indeterminate value, contained in the outputs from the specified shift register, setting a state value of “1”; and an output verifier to verify the masked output results of the specified shift register, from which output results the indeterminate value is excluded; wherein said mask comprises: a decoder comprising a plurality of outputs as many as the number of last sequential circuits in said plurality of shift registers, a plurality of second sequential circuits, associated one with each of said plurality outputs from the said decoder, each of said plurality of second sequential circuits has an output value determined in accordance with the associated output from said decoder, and a plurality of logical arithmetic units, associated one with each of said plurality of second sequential circuits, each of said plurality of logical arithmetic units performing an arithmetic operation on the output value from the associated second sequential circuits and an output value from one of the last sequential circuits with the associated second sequential circuit.