A substrate bias potential generator for biasing a semiconductor substrate to a predetermined potential includes first and second substrate bias generating circuits which operate alternatively according to the potential of the substrate, whereby consumption of power in the substrate bias potential generator is reduced. The alternative operation of the bias generating circuits each activated by a pulse signal train is performed by using a first insulated gate transistor having a gate electrode connected to the semiconductor substrate, a second insulated gate transistor having a gate electrode for receiving the reference potential, an amplifier for differentially amplifying outputs of the first and second insulated gate transistors, an insulated gate transistor for charging an output of the amplifier to a predetermined potential when the amplifier is activated, and a circuit for transmitting the output of the differential amplifier to the first and second bias potential generating circuits. The differential amplifier is activated in response to an activation signal of a pulse train whereby an activation signal corresponding to the pulse train is transmitted to either substrate bias potential generating circuit.