PURPOSE: To estimate a two-dimentional rainfall strength distribution which will be created several hours later, by a method wherein, from a time system of a two- dimentional strength distribution of rain falling for several fours, a condition on the movement of distribution and an influence of configuration of the ground are extracted, and said information is used.

CONSTITUTION: A two-dimentional rainfall strength distribution f(x, y) is observed in a digital amount by a rainfall radar, where an x-axis is a direction extending east and west, an y-axis a direction extending south and north, and f(x, y) is a rainfall per unit hour at a point (x, y). If a distribution, creeted k×Δt time later from a reference time, is represented in fk(x, y) and moves by u in the east and west direction and by v in the sound and north direction during a Δt time, (u, v) can be estimated by finding a value minimizing an equalityI, (u, v), which minimize an equality II, are found by taking a rainfall variation rate w(x, y) at each point into consideration, and w(x, y) is further corrected with the result. The desired (u, v) and w(x, y) are found by repeating and smoothing the operations. The use of the result and an equality III permits the estimation of a ditribution fL (x, y) which is created several hours later after a current distribution fk(x, y).