In a semiconductor integrated circuit device comprising a CMOS circuit, the CMOS circuit operating at a high speed, consuming a small amount of power, is achieved. In particular, acceleration of the operating speed under low voltage is achieved. The semiconductor integrated circuit device of the invention comprises a main circuit including a CMOS circuit, a changeover circuit, a substrate bias control circuit and a switching circuit and, in accordance with a changing signal from the changeover circuit, switches states of a substrate of a MOS transistor of the main circuit between a state in which normal supply voltage as well as ground voltage are applied and a state in which forward bias is applied. The changeover circuit detects a drop in supply voltage, etc. and outputs changing signals.