To provide a multipipe protecting circuit of an electronic ballast, which quickly and surely cut off an abnormal fluorescent lamp circuit to protect a normal fluorescent lamp, while reducing the cost.

All fluorescent lamp circuits are connected to a conversion circuit in parallel. A current signal is taken out of each fluorescent lamp circuit, which is sent to a failure detecting circuit. Then calculation at an AND gate is made, together with a control signal to operate a chip switch of the conversion circuit, while an abnormal fluorescent lamp is detected indirectly. The fluorescent lamp circuit is automatically cut off, thus providing a ballast to protect a multiple fluorescent lamps at a minimum cost.