Lotus-type metal is a newly developed porous material containing numerous unidirectional cylindrical pores, which is produced by unidirectional solidification of the melted metal in a pressurized gas atmosphere. To effectively employ lotus-type porous metal as a heat sink, it is necessary to clarify not only the heat conductivity but also the fluid permeability, which depends upon the porous structure. The Darcy permeability was measured from the water permeation test of lotus-type porous copper under extremely low pressure conditions. The true permeability determined by accounting for the effect of the penetration ratio of pores was well evaluated by applying the Hagen-Poiseuille equation to the fluid flow, using the fourth root mean fourth power value as the average pore diameter in view of the pore size distribution. Moreover, the effect of the pore size distribution on the permeability was analyzed in the case of the log-normal distribution, and it was shown that the permeability increased with increasing standard deviation.