An ON failure detecting apparatus detects an ON failure of an FET T1 of a power supply circuit which includes the FET T1 disposed between a battery E and a load RL, and controls driving and stopping of the load RL. The ON failure detecting apparatus includes a driver circuit 1 that supplies a driving voltage VD for switching ON and OFF of the FET T1 to a gate of the FET T1, a gate resistor Rg that is provided between the driver circuit 1 and a gate of the FET T1, and an ON failure determining circuit 11 that detects whether a voltage drop in the gate resistor Rg exceeds a predetermined value, and determines that an ON failure occurs in the FET T1, when the voltage drop in the gate resistor Rg exceeds the predetermined value.