In a power on reset (POR) circuit, when power is turned on, an output signal of an inverter attains an H level and an N channel MOS transistor is rendered conductive. The potential of an input node of the inverter becomes a potential of a power supply voltage divided by a conductive resistance value R1 of a P channel MOS transistor and a conductive resistance value R2 of an N channel MOS transistor. Assuming that the threshold voltage of the inverter is 0.8 V and R1:R2=2:3, then the power supply voltage Vres at the time when signal POR# inverts its level becomes 1.33 V. Thus, this POR circuit can reliably be utilized even in a product designed to operate with 1.5 V incorporating a MOS transistor having a threshold voltage of 0.8 V.