A block for constructing a breakwater comprises a front wall in which horizontal projections are formed with transverse rectangular-shaped holes therebetween. Side walls extend from both ends of the front wall. A partition wall in which multiple holes are formed to communicate with the transverse rectangular-shaped holes is jointed to the side walls at both ends and spaced from the front wall. A detention area is defined by the front wall, partition wall and side walls for receiving the overflow waves through the transverse rectangular-shaped holes in the front wall.