The scour in front of breakwaters is sometimes the cause of scattering of armour blocks, foot protection blocks and rubbles which directly affects to the stability of breakwaters. Thus it is very important to find out the effective protection methods against scour.
　In this paper, scour in front of breakwaters by diagonal incident waves is studied and effectiveness of several protection methods which utilize the hydraulic chatacteristics of the scour are tested.
　It is found that a method to spread gravels at the toe of rubble mound of composite type breakwaters protects scour effectively and this is reasonably explained from hydraulic stand points.