In a semiconductor device having a data input buffer capable of inputting write data to each of memory units, the data input buffer is changed from an inactive state to an active state after the reception of instruction for a write operation effected on the memory unit. The data input buffer is a differential input buffer having interface specs based on SSTL, for example, which is brought to an active state by the turning on of a power switch to thereby cause a through current to flow and receives a signal therein while immediately following a small change in small-amplitude signal. Since the input buffer is brought to the active state only when the write operation's instruction for the memory unit is provided, the data input buffer is rendered inactive in advance, before the instruction for the write operation is provided, whereby wasteful power consumption is reduced. In another aspect, power consumption is reduced by changing from the active to the inactive state in a time period from a write command issuing to a next command issuing.