A model experiment of irregular wave action on a caisson breakwater armored with concrete blocks has been carried out in order to investigate the following:
　(1) wave reflection and transmission
　(2) stability of wave dissipating concrete blocks
　(3) wave forces on a caisson.
　The experiment on wave reflection is performed in the depth of 39 cm. Reflection coefficient is indicated for the range of relative water depth from 0.081 to 0.436. Transmitted waves due to incident waves of 2.5 seconds are tested in the depth of 39, 42, and 48 cm. The relation between relative height of breakwater and transmission coefficient is presented.
　The experiment on stability of wave dissipating concrete blocks is performed in the same water depth as the experiment on transmitted waves. The KD-value in Hudson's formula in terms of the percentage damage are investigated for two different types of concrete blocks.
　The stability of caisson against irregular waves of 2.5 seconds in the significant wave period is tested in the depth of 39 and 42 cm. Wave forces are estimated on the basis of critical wave height and critical weight of caisson against sliding.