To provide a precise signal detector where its circuit configuration is simplified in the detection of real component I and an imaginary component Q of a complex signal and I and Q components with the same phase can be obtained and to enhance the performance of an ultrasonic wave diagnostic device using the signal detector.

The signal detector acquires samples of a received signal at each timing tn deviated from the phase of its carrier by π/4 and a multiple of an odd number m of π/2. A cosine code operation section 174 applies code operation to each sampling value S(tn)=I(tn)cosθ(tn)-Q(tn)sinθ(tn) outputted from a data rate conversion section 170 where a sign of a coefficient of the I(tn) is made constant. Thus, that is, the Q(tn) has a coefficient whose sign is inverted for each m π/2. That is, the term with respect to the Q(tn) is a vibration term. The I(tn) can be acquired by filtering out the vibration term with an LPF 184. On the other hand, a sine code operation section 176 applies an operation to the same S(tn) to bring the I(tn) into the vibration term and an LPF 194 outputs the Q(tn) at the same phase with that of the I(tn).