A new type of caisson with a high stability against violent wave attacks has been developed and is called as a multi-celler caisson in the present paper. This caisson consists of triple-layered slopes with the rear end bended upwardlly. The spaces formed between each slopes are opened at the both ends of the vertical front and the horizontal top of caisson.
　A series of model experiments has been conducted to investigate the following hydraulic aspects of the multi-celler caisson:
(1) wave reflection and transmission due to random waves,
(2) resultant wave forces on the caisson and stability against sliding,
(3) wave forces on structural members of the caisson.
　In this paper, the design wave force formulae are proposed for the stability examination and the strength calculation of the structural members. According to these formulae, an example of trial design for the stability of the multi-celler caisson is presented and compaired with the ordinary caissons. It turns out that the multi-celler caisson has great possibility as the outer sea breakwaters.