A booster circuit including a precharge capacitor (C.sub.2), a precharge driver circuit (20) having a first bootstrap circuit (C.sub.59, Q.sub.58, Q.sub.61) and precharging a voltage to the precharge capacitor in a reset mode, and an output driver circuit (19) having a switching circuit (Q.sub.21) for cutting off the output of the precharged voltage of the precharged capacitor in the reset mode and a second bootstrap circuit driving the switching circuit in an operation mode. The booster circuit further includes an additional switching circuit (Q.sub.1) for outputting a voltage to be superimposed onto the precharge voltage in the operation mode.The booster circuit may be applicable to a dynamic semiconductor memory device, for boosting a voltage of a word line at a high speed and for improving integration.