An LSI has bidirectional buffers connected to a boundary scan circuit. The boundary scan circuit 12 has asynchronous setting circuits for setting each bidirectional buffer to input mode or output mode. The bidirectional buffers are asynchronously and uniformly set to output mode to detect a logic error. If there is no logic error, input/output terminals which are respectively connected to the bidirectional buffers are integrated and the bidirectional buffers are asynchronously and uniformly fixed to input mode. After setting a set value for setting a desired enable state to the boundary scan circuit, the uniform input mode is released asynchronously. Then, the boundary scan circuit implements DC test.