The method and circuit find the reciprocal value of an input vector signal. The level of the input vector signal (X + jY) is first reduced to (X + jY)/ 2 in an overflow-preventing circuit. A power calculating circuit squares and adds the components of the level-reduced input vector signal, thereby obtaining a power value (X² + Y²)/2 . An initial value of a tap value (K), which represents the reciprocal value to be found, is multiplied twice by a multiplying circuit, thereby obtaining K²(X + Y)²/2 . Further, a differential circuit obtains an error signal (ΔK) = 1/2 - K²(X² + Y²)/2 by subtraction from a reference value. An updating circuit updates the tap value (K) by adding to it the error signal (ΔK). A loop consisting of the multiplication of the tap value, differential operation, and updating of the tap value, is repeated until the error signal (ΔK) is reduced to less than or equal to a predetermined value. The tap value (K) thus obtained is the desired reciprocal value 1/√(X² + Y²) of the amplitude of the input vector signal. The method and circuit can be used for amplitude normalization in a modem, for example.