PURPOSE: To prevent a semiconductor integrated circuit element from deteriorating in operation performance even if the voltage of an external power source drops by supplying the electric power from an internal voltage power source only when the internal source voltage of an internal power source drops below the level where the drop of the internal source voltage is detected.

CONSTITUTION: If the external source voltage VCC drops to become equal to an internal voltage VINT, a transistor(TR) 3 turns ON. At this time, the values of resistors R4 and R5 are so set that the level at a connection point N1 equals to the level at a connection point N2. Therefore, an internal power circuit for external source voltage dropping time operates if the internal source voltage VINT drops even slightly on condition that the external source voltage VCC equals the internal source voltage VINT, but does not operates unless the internal voltage VINT drops greatly as the external voltage rises. Consequently, a deficiency of the current capacity of a TR 1 when the external voltage VCC drops is compensated by a TR 2 to prevent the internal source voltage VINT from oscillating where the external source voltage VCC is high.