Voltage detecting circuit for detecting a voltage state of a voltage source includes a first route, a second route and a comparator. First route is between a voltage source and a ground voltage, having a first current source and a first MOS resistor, connected in series at a first node with outputting a first voltage. Second route is between the voltage source and the ground voltage, having a second current source and a second MOS resistor, connected in series at a second node with outputting a second voltage. The comparator receives the first voltage and the second voltage and outputs the voltage state. The comparator includes a current offset circuit for producing an offset voltage to the second voltage. A voltage comparing unit compares the first voltage and the second voltage with offset by a voltage difference, and outputs the voltage state according to variance of the voltage difference.