Abstract: To improve the stability of numerical calculation for finding a continuous phase of a pair of signal sequences, provided is a signal processing device (10) wherein a spline calculation unit (13) approximates a pair of signal sequences acquired by a signal acquisition unit (11) by spline functions, and an unwrapping processing unit (18) performs phase unwrapping using polynomials. The phase unwrapping is performed according to a procedure where a polynomial-sequence calculation unit (15) calculates a polynomial sequence by applying a Euclidean algorithm to the polynomials of the spline functions, a sign counting unit (17) checks the number of changes of the sign of a numerical sequence formed by arranging the values of the polynomial sequence in each subinterval where the phase is found, and an unwrapping processing unit (18) determines an indefinite portion that is an integral multiple of ? on the basis of the number of changes.