A dual-port type semiconductor memory device having a serial data input and output circuit (200) provided outside of a memory cell array (1) and operable for high-speed serial data input and output of data in addition to random data access. The semiconductor memory device includes a single decoding circuit (5) triggering at least one gate for transferring data to be stored into or read from the memory cell array in a random data access mode and setting a single bit to a corresponding shift register (24) in the serial data input and output circuit in a serial data input and output operation mode. Preferably, the decoding circuit is operated only during a time for operatively connecting bit lines and latch circuits in the serial data input and output circuit in the serial data input and output operation mode. The serial data input and output circuit is operable independently from the memory cell array, except during the time for operatively connecting the bit lines and the latch circuits through transfer gates, for serially inputting data to or outputting data from the latch circuits through serial data bus by sequentially triggering the serial gates from a certain gate designated by the corresponding shift register in response to the decoding circuit.