PURPOSE: To obtain an inexpensive, high-speed testing function by providing a selector which fetches a microinstruction in a general register externally and transfers its output to a microinstruction register for a test of a microprocessor.

CONSTITUTION: When a test control signal D is applied, a special address is set in a control storage address register 9, a branch instruction is executed, and the control is transferred to a branch-destination program. The branch-destination program transfers external microinstructions to the general register 8 and passes the control to the microinstructions. A specific area of a control storage part 5 is addressed at the register 9 by the execution of the branch instruction after the microinstruction is stored to output a control storage switching signal H; and selectors 11 and 13, a buffer 12, and the selector in a counter 10 are operated. The microprogram stored in the general register 8 is read out to a microinstruction register 6 and executed. Thus, the internal test is taken without providing externally a microinstruction sending device for the test.