In a substrate potential generator, a substrate potential is supplied by a substrate potential supplier controlled by a substrate potential detector. The substrate potential detector sends a setting signal having a hysteresis characteristic relative to the substrate potential. That is, the setting signal is higher when the substrate potential supplier is stopped than when the substrate potential supplier is activated or when negative charges are injected into the substrate potential. Thus, the operation of the substrate potential supplier is stopped after the substrate potential becomes lower than the lower setting potential when the substrate potential supplier is activated, while the operation of the substrate potential supplier is started after the substrate potential becomes higher than the upper setting potential after the operation of the substrate potential supplier is stopped. Therefore, the starting and stopping of the substrate potential supplier is not repeated so frequently, so that the dissipating charge and discharge currents accompanied with the starting and stopping will not be enhanced wastefully.