A constant voltage generating device comprises a reference voltage generating circuit, a constant-current circuit unit and a current-to-voltage converting circuit unit. The reference voltage generating circuit generates a desired reference voltage. The constant-current circuit unit comprises a differential error amplifier to which the reference voltage generated by the reference voltage generating circuit is input as a reference potential, a first current controlling MOS transistor having a gate electrode to which an output of the differential amplifier is input, and a standard resistor serially connected to the first current controlling MOS transistor. The constant-current circuit unit generates a reference current to control a differential amplifier so that a constant current can be caused to flow therethrough. The current-to-voltage converting unit comprises a second current controlling MOS transistor constituting a current mirror together with the first current controlling MOS transistor of the constant-current circuit unit, and a current-to-voltage converting MOS transistor serially connected to the second current controlling MOS transistor and constituting a current mirror together with an active element unit current controlling MOS transistor of the differential amplifier.