A sliding mode control introducing a conditional integrator is proposed for the robust output regulation of DC/DC buck converter. Based on the input-output linearization from the state-space averaged model of DC/DC Buck converter, the robust output regulation problem of the converter can be converted into a robust stabilization problem of an augmented system consisting of the given buck converter and the internal model by introducing a proper conditional integrator. The procedures include the design of an internal model with conditional integrator and a sliding mode robust controller. Also the closed-loop robust stability is theoretically analyzed. Finally, the effectiveness of the proposed converter is verified in comparing the proposed controller performance with open-loop control, single-loop proportional plus integral control and double loop proportional plus integral control under load disturbances and supply voltage variations.