PURPOSE: To obtain a detection output with high reliability, by detecting whether or not a power supply voltag is decreased from a margin voltage having a good relative accuracy to a memory cell holding voltage of a static RAM circuit.

CONSTITUTION: In figure, 30 is a memory cell of a static RAM circuit and a flip-flop circuit 34 of the same connection is constituted and the relation of the size of each transistor (TR) is set as T1'>T2'=T1=T2, T3'<T4'T3=T4. Thus, the holding voltage of a power-down write data is increased slightly (α) higher than that of the memory cell 30 in the flip-flop circuit 34 because of the unbalance of the potential of nodes A and B. Thus, when a standby power supply voltage VSB is lower than the data holding voltage of the memory cell 30 having a margin voltage α, the TRT5 is inverted off, the node A is inverted to the high level and the TRT4' is inverted on, the node B is inverted to the low level, allowing to attain the detection of power-down.