PURPOSE: To obtain an excellent characteristic at a low frequency band by using two full band pass filters having a phase difference of 90°.

CONSTITUTION: Full band pass digital filters A1,B2 having a phase difference of 90° have a phase characteristic θ expressed in equation I. In the equation I, T indicates a sampling period [n'=n/2 for an even number of (n) and n':(n+1)/2 for an odd number of (n), Pi indicates a filter coefficient (real number), ω is a frequency of an input signal and (m) is a positive weight variable other than the unity in the relation of 2mω/P1<1 when ωT is sufficiently smaller than the unity and in the relation of -P1ωT2/2m<1 is nearly equal to π. Each filter consists of cascade connection of n'-stages of sections, and each section is provided with a multiplier 11, an adder 12, a register 13, multipliers 14,15 and an adder 16 and the 90° digital phase shifter is formed by using the two filters.