Constant voltage regulator includes an output terminal, an output transistor, an output voltage controller, and a switching circuit. The output terminal outputs a first voltage to an external host apparatus. The output transistor outputs a current in accordance with a signal. The output voltage controller amplifies a difference between a reference voltage and a feedback voltage produced in proportion to the first voltage, and outputs the amplified voltage to the control gate of the output transistor. The switching circuit selectively outputs one of a second voltage greater than the first voltage and a third voltage greater than the second voltage to a substrate gate of the output transistor in response to an operation mode switchable between a first mode in which an output current to the external host apparatus is smaller than a predetermined value and a second mode in which the output current is greater than the predetermined value.