Abstract: The operation code of a user instruction is supplied to a control storage device and a second entry control storage device, through a selector. The control storage device produces a start microinstruction of a microprogram corresponding to a user instruction specified by the operation code. At the same time, the second entry control storage device produces an address for specifying a memory location of the control device where a second microinstruction to be executed following the microprogram is stored. At the next step, the start microinstruction is executed and at the same time the second address produced is supplied to the control storage device and the second microinstruction of the microprogram is read out. In the succeeding steps, either the address of an address register or a branch address of a microinstruction data register is supplied to the control storage device through the selector, thereby to read out the microinstruction.