A constant-voltage circuit includes a switching regulator and a series regulator each configured to start operating according to a selection signal input from outside the integrated circuit for exclusively causing one of the switching or the series regulators to operate, convert an input voltage to a rated voltage and to output the voltage to an output terminal. The switching regulator performs a soft-start operation for raising an output voltage to the rated voltage at a constant speed when starting operation. The series regulator regulates a current so that an output current does not exceed a predetermined value, simultaneously starts operating when the switching regulator starts operating, and automatically stops outputting a current to the output terminal when the soft-start operation is completed. A semiconductor apparatus provided with a predetermined function includes the constant-voltage circuit. A control method controls the constant-voltage circuit.