Abstract: A ultrasonic Doppler blood flow monitoring system is disclosed. The ultrasonic Doppler blood flow monitoring system comprises: a catheter to be inserted into a blood vessel; at least two ultrasonic transducers provided on the catheter and arranged for transmitting ultrasonic waves toward blood flow at two different angles of incidence with respect to blood flow direction and then receiving the reflected ultrasonic waves, to obtain two ultrasonic Doppler shift frequency signals due to blood flow; and a calculating section for calculating the velocity of flowing blood on the basis of the received Doppler shift frequency signals. Therefore, even if the catheter is not placed in parallel to the blood flow direction at a bent portion of a blood vessel, it is possible to continuously calculate the velocity of flowing blood accurately.