PURPOSE: To diagnose a fault of shift buses in every circuit block unit, by providing each circuit block with a shift bus input switching circuit, and disconnecting a mode setting register from the shift bus during fault-position bracketing operation.

CONSTITUTION: When a fault is detected over buses 201W208 and the fault position is bracketed by circuit blocksIand II, bits (a) and (b) of a mode setting register 1 are set to logic 1. Then, a mode switching signal 103 is given logic 1 to close NAND circuits 32 and 33, stopping the supply of a clock signal 102 to the register 1. The output of an NAND circuit 31 has logic 1 and a shift bus switching circuit 2 is switched to send the output 205 of the blockIfor every shift bus to the output 208 of the circuit 2. Further, an NAND circuit 36 closes an NAND circuit 38, so the supply of a shift-out signal 202 in every shift bus unit of the blockIis cut off, thereby disconnecting the register 1 from the shift buses.