[en] Japan Atomic Energy Agency (JAEA) has been performing the development and design of Water-cooled ceramic/Be type test blanket module (TBM). The TBM has a box structure that consists of six plates, and circler or rectangular cooling flow channels are built in the plates. In the side wall, parallel flow channels are considered. Two parallel large channels that act as a manifold are connected by small parallel channels. In the cooling channels structure, the flow distribution probably arises from the pressure drop in the channels. The purpose in this study is to clarify the water flow distribution in the side wall and design the cooling channels structure so that structural material of the side wall can be kept under the allowable temperature. The calculation of maximum temperature of the structural material is performed by a one dimensional thermal calculation, and the minimum flow rate of the coolant to keep the structural material under the allowable temperature is estimated. The flow distribution among parallel flow channels is estimated by the calculation of the pressure drop, and then the cooling channels structure is designed. The design is verified by the water flow experiment using the mockup made of the vinyl chloride pipe. For the verified design, the mockup made of F82H is manufactured, and the water flow distribution and the pressure drop are measured in the water flow experiments. The result shows that decrease rate of the water flow to the average is 12% in maximum. This is sufficient in the view of the heat removal capability. It is also confirmed that the calculated pressure drop corresponds to the measured one. From these results, the design for the cooling channels structure in the side wall is established so that enough water flow to cool the structural material is kept. (author)