PURPOSE: To reduce the circuit scale by providing an operation clock switch means which switches the operation clock of a counter in response to the clock of the input/output data to be applied to a RAM.

CONSTITUTION: An operation clock switch circuit 1 outputs an operation clock 4 to a counter 2 and switches (n) types of clocks 4 in response to the clock of the input/output data applied to a RAM 3. In other words, the circuit 1 switches the clocks 4 with change of the clock of the input/output data. An address 5 is outputted from the single counter 2 and therefore the continuous changes are kept despite the change of the clock 4. Thus the circuit scale is reduced and the access speed can be slowed down for the element of the RAM 3.