Long period waves have often caused the interruption of cargo handling and break of mooring lines in harbors bordering to the Pacific Ocean and Japan Sea. Several countermeasures against the long period wave disturbance have been proposed since the authors revealed the mechanism of their transformation in harbors in 1996. One of such countermeasures is “long period wave absorber” mainly composed of wide rubble layer behind breakwaters to prevent the amplification of long period waves by multi-reflection in harbors.
　The numerical analysis for applicability of various type long period wave absorbing countermeasures has not been fully carried out though several types of long period wave countermeasures are proposed. The paper describes the variation of long period wave reflection coefficient for the four rubble mound type absorbers and the eddy absorbing type countermeasure. Secondly, a field observation of long period wave reflection for a rubble detached breakwater installed behind the main caisson-type breakwater is introduced.
　The main conclusions are as follows;
ⅰ) The long period wave reflection coefficient of various rubble layer type countermeasures is evaluated in a diagram for B/L(width of rubble layer/ wave length).
ⅱ)The reflection coefficient of eddy absorbing type countermeasure becomes less than 0.6 for a specified wave period.
ⅲ)The observed wave reflection coefficient in the field test agrees well to the estimated one.