The finite difference methods called "split-operator approach" generically are often applied to numerical computations of 2-dimensional convective-dispersion equation for shallow sea. Of the methods, the method of characteristics using shifting particles (called shifting particles method in this paper) has accurate algorithm especially for calculating the convection term in the equation. On this method, a number of particles having the concentration at which each particle exists are set over computation region. Shifting the particles to the direction of velocity vector, the variation of the concentration for each particle on its characteristic line is calculated. In this paper, a method for applying shifting particles method to 2-dimensional convective-dispersion equation may be described. For the imaginary computation region that is a regular square (10 km×10 km), the relationship between number of particles set over computation region and computation accuracy is studied comparing the numerical solutions with the analytical. The results indicate that it is necessary to set three particles on one computation grid, and that more than four particles may realize accurate computation performance for more complex computation regions such as practical problems.