There is provided a semiconductor device of low power consumption and high reliability having DTMOS' and substrate-bias variable transistors, and portable electronic equipment using the semiconductor device. On a semiconductor substrate (11), trilayer well regions (12, 14, 16; 13, 15, 16) are formed, and DTMOS' (29, 30) and substrate-bias variable transistors (27, 28) are provided in the shallow well regions (16, 17). Large-width device isolation regions (181, 182, 183) are provided at boundaries forming PNP, NPN or NPNP structures, where a small-width device isolation region (18) is provided on condition that well regions on both sides are of an identical conductive type. Thus, a plurality of well regions of individual conductive types where substrate-bias variable transistors (27, 28) of individual conductive types are provided can be made electrically independent of one another, allowing the power consumption to be reduced. Besides, the latch-up phenomenon can be suppressed.