A semiconductor integrated circuit including an internal voltage step down circuit exhibits a first voltage characteristic I having substantially no dependence on an external power supply voltage VEXT and holding an internal power supply voltage VINT at a constant voltage VA if the external power supply voltage VEXT is in the range between two predetermined values V1 and V2. On and after the external power supply voltage VEXT exceeds the predetermined value V2, the circuit exhibits a second voltage characteristic II, in which the internal power supply voltage VINT varies from the constant voltage VA in accordance with the external power supply voltage VEXT, during a non-accelerated test (operation margin certification test). On the other hand, during the accelerated test, the circuit exhibits a third voltage characteristic III in which the internal power supply voltage VINT reaches a certain voltage VB (>VA) and goes on increasing from VB in accordance with the external power supply voltage VEXT. Thus, during the operation margin certification test, the internal power supply voltage VINT exhibits continuously varying characteristics I and II and the operation of the circuit can be assured in the voltage range from VA to VB. During the accelerated test, a voltage can be increased at a sufficiently high rate owing to the characteristic III.