A voltage restraint device which is applied in an operational amplifier comprises a first transistor; the trans-voltage of a gate and a source approaches to a certain voltage and the trans-voltage of a drain and the source is not equal to zero, thereby leading to a larger substrate electric current. The voltage restraint device comprises a second transistor; the source is coupled to the drain of the first transistor; the gate receives bias signals, which leads the second transistor to be operated in a saturation region and leads the source voltage of the second transistor is equal to the gap between the bias signals and the threshold voltage of the second transistor; therefore, the electric current of the substrate can be reduced by reducing the trans-voltage of the drain of the first transistor and the source.