A car obstacle monitoring system which will not erroneously detect a distant obstacle as an obstacle around a car and can avoid an erroneous detection by relatively simple control even though it is arranged so as to emit ultrasonic waves of a single frequency. A control circuit 12 controls a right sonar 1, a left sonar 4 and center sonars 2 and 3 provided on the right side, on the left side and at the center of a car 5, respectively. The control circuit 12 has a timing element and a discriminating element. The timing element generates three emission modes including a first emission mode in which the right sonar 1 and the sonar 3 emit ultrasonic waves in synchronism, a second emission mode in which the left sonar 4 and the center sonar 2 emit ultrasonic waves in synchronism, and a third emission mode in which the right sonar 1 and the left sonar 4 emit ultrasonic waves in synchronism repeatedly without overlapping each other. The discriminating element finalizes a discrimination that an obstacle has been detected when reflected waves are received consecutively by plural times in predetermined sampling times.