A semiconductor device is equipped with a step-up circuit having a series of multiple charge pump units. Each of the units has a well separation type MOS transistor. The separation well of the transistor is coupled to a high potential so as to form double reverse biases between the N-type well and a P-type substrate and between the N-type well and a P-type well. This permits the threshold Vth of the MOS transistor to be held at low level. The units are provided with a clock whose current supply capability is limited until a predetermined condition (that a predetermined period of time has elapsed after the onset of the step-up circuit by a startup signal or that the output voltage has reached a predetermined level). This limitation of the clock facilitates suppression of power consumption by the step-up circuit during a startup, thereby reducing changes in amplitude of a supply voltage.