PURPOSE: To reduce the circuit scale of a blood flow imaging system and to also reduce cost by performing the LOG compression of an imaginary component signal and a real component signal by the first and second table memories and outputting a mean blood flow velocity signal by the third table memory.

CONSTITUTION: An LOG table is formed to the first table memory ROM 32a and the output Im{C(τ)} of the first adder 31a is taken in an address to output the LOG compression result of an imaginary component signal Im{(C(τ)}. The LOG table is also formed to the second ROM 32b and the output Re{C(τ)} of the second adder 31b is taken in an address to output the LOG compression result of this real component part signal Re{C(τ)}. An ANTI-LOG (antilogarithmic) table is formed to the third ROM 33 and the outputs Y, X of the first and second ROM 32a, 32b are taken in addresses to output a mean blood flow velocity signal fd. By this method, hardware constitution for opera tional processing is simplified and the circuit scale of the blood flow imaging system is reduced and cost reduction is achieved.