The present invention is to provide a velocity measuring method and a velocity measuring device for carrying out the method. The velocity measuring method includes: a step (S4) for expanding N time series signals by using 0-th to (N−1)-th degree discrete Legendre function as a base; a step (S5) for calculating 2n-th degree complex expansion coefficient by multiplying a linear combination of a (2n−1)-th degree expansion coefficient and a ( 2n+1)-th degree expansion coefficient by an imaginary unit and then linearly combining the result and a 2n-th degree expansion coefficient, and calculating a (2n+1)-th degree complex expansion coefficient by multiplying the (2n+1)-th degree expansion coefficient by an imaginary unit and then linearly combining the result, the 2n-th degree expansion coefficient and a (2n+2)-th degree expansion coefficient; a degree decision step (S4) for determining the degree m of a coefficient having the maximum absolute value among the complex expansion coefficients; and a step (S8) for calculating a signed velocity signal concerning a moving reflection object from a ratio of square sums of the expansion coefficients or complex expansion coefficients corresponding to the degree m.