The seawater exchangeable breakwater developed by Kitsukawa et al is one of the types that have water drawing mechanism applying wave breaking. Floating body units set in it move up and down with tidal level change, and keep effective water depth for wave breaking at all time, therefore sea side water can be drawn into harbor side efficiently. However, several important structural parameters are not known enough for practical design. In this study, we investigated appropriate values of clearance between floating body and side walls, opening area ratio of discharge pipe sectional area to submerged back wall area, and tested a difference in materials of floating body. Results are as follows: (a) Opening area ratio should be about 8% to perform efficient flow rate and low wave transmission toward harbor side. (b) Clearance should be about 100mm to realize smooth movement of floating body. (c) Kinds of material do not affected to capability of water exchange.