A fault detection circuit connects to and determines the occurrence of failure in an inverter circuit. The inverter circuit comprises three outputs to connect three groups of lamps respectively, and the fault detection circuit comprises a magnetic unit and a signal detection unit. The magnetic unit comprises first, second and third flux generating windings electrically connected to the three outputs of the inverter circuit, and a flux detection winding. If no fault occurs on the outputs of the inverter circuit, total flux generated by the flux generating windings is cancelled out. As long as any fault occurs on the outputs of the inverter circuit, flux generated by the flux generating windings cannot be canceled out, and the flux detection winding is electromagnetically coupled accordingly and driven by the generated flux to output a coupling signal, based on which the signal detection unit generates an alert signal accordingly.