PURPOSE: To cope with the switch to a redundant complementary data line by providing a saving signal line to which an input and/or output terminal of each bit of a shift register is selectively connected according to a prescribed selection control signal.

CONSTITUTION: The saving signal lines BI0, BO0 - BI3, BO3 to which the optional bits are selectively connected according to a prescribed selection control signal are added to the shift registers SR0 - SR3 which are used for a pointer PNT of a serial memory, etc. Then the redundant shift registers RR0 - RR3 set in accordance with the redundant complementary data lines Dr0 - Dr3, for example, are connected to those saving signal lines. Thus the optional bits of the registers SR0 - SR3 are connected to the registers RR0 - RR3 via the saving signal lines BI0, BO0 - BI3, BO3. Then it is possible to cope with the switch to the redundant complementary data lines.