A first detector compares an electric signal to be monitored with a first threshold. A second detector compares the electric signal with a second threshold. A first memory stores setting data of the first threshold. A second memory stores setting data of the second threshold. An interface circuit receives data from an external processor, and writes the data thus received to the first memory and the second memory. The protection circuit is configured such that data writing to the first memory is possible only when a predetermined condition is satisfied.