Measurements using a series of laboratory experiments are carried out to investigate the effects of a submerged structure in protecting a rubble mound breakwater. In this study, a rectangular submerged structure is installed in front of a rubble mound breakwater, and irregular waves are generated by a piston-type wave maker installed at the end of a channel 32.5 m long, 0.6 m wide, and 1.1 m deep. In laboratory experiments, the effects of heights of the submerged structure on the transmission coefficients and run-up and run-down heights are evaluated. The role of the submerged structure on the stability of a breakwater is also investigated.