In a semiconductor integrated circuit for a DC--DC converter, an nMOS-type transistor Qn of a CMOS inverter 1c constituting a driver 1 is electrically floated from a substrate 12 through an n-type well region 11. Thus, the nMOS-type transistor Qn is electrically insulated from other transistors such as an npn-type transistor Q1 and an L-pnp-type transistor Q2 constituting a feedback control system 9 through the n-type well region 11. Stable operation is performed with a minute current without producing a malfunction caused by the influence of a parasitic current, even if the drain potential of an nMOS-type transistor is reduced to the ground potential or lower at the time of switching by a driver constituted from a CMOS inverter, to facilitate lower power consumption and higher efficiency, and also to eliminate a constraint on layout design of components.