The module construction system is established as an efficient production system in a shipyard. Because an efficiency and productivity of the module construction system are depends on block division plans, it is necessary to make proper block plans. A lot of design support systems for shipbuilding have been recently developed, but it seems that there is no effective support system for a block division planning. Therefore, a computer aided block division planning is a problem to be solved for shipbuilding CIM. In order to develop a computer aided block division planning system, this paper introduces a concept of candidate seam and shows that information on joint plays a significant role in block division planning. In this research, a computer aided block division planning system is introduced, and this system gives two processes with user interface; -A block designer specifies information on joint of block seams and distributes structural members to each block to design blocks. - A system generates information on joint at any position at cutting apart designed structural members. - A system proposes some block division plans from specified block seams. Above-mentioned functions for block division planning are implemented into the shipbuilding CIM system of authors. In order to select better division block, this system evaluates designed blocks by using product data and constraints of a facility. A few outputs from the block division planning system is shown in this paper.