To control the side lobe characteristic of a directional pattern by calculating a one-dimensional frequency response corresponding to an impulse response, calculating a two-dimensional frequency response between the normalized time frequency and the normalized space frequency, calculating an impulse response via the inverse Fourier transform, and calculating the coefficient of an FIR type digital filter, based on the impulse response.

An FIR type digital filter coefficient computing element 13 calculates a one-dimensional frequency response based on the impulse response of a one- dimensional narrow and low bond pass digital filter of the normalized time frequency having the impulse response length M smaller than an element number N of an array antenna 100. Then a two-dimensional frequency response is calculated by performing the frequency conversion between the normalized time frequency and the normalized space frequency, and an impulse response of the two-dimensional frequency response is calculated by the inverse Fourier transform. The side lobe of the impulse response is cut at 0, based on the element number N and a tap number K to calculate the coefficient of an FIR type digital filter.