The effect of a porous structure formed after dissolution of fine particles dispersed in a coating layer was investigated on the release rate of core particle by random walk simulation. As a result, the release rate was described by the structure coefficient which increases with the ratio of contact to the surface area. The ratio was independent of the average size of soluble particles and it increased with an increasing volume fraction of soluble particles, with an increasing geometric standard deviation of particle size at the low volume fraction, and with a decreasing standard deviation at the high fraction. Then, the simple correlation was established between the structure coefficient and porous-structure parameters, which should be useful for the design of the release pattern.