The present invention relates to a synchronous semiconductor memory device with double data rate, and more particularly, to a synchronous semiconductor memory device for inputting and outputting data using a free-running clock and inserting a preamble indicative of start of data into the outputted data. A semiconductor memory device of the present invention receives a data read command from the exterior of the memory device in response to a predetermined clock signal inputted from the exterior, and outputting data including a preamble in response to the clock signal.