PURPOSE:To detect the abnormality of an electromagnetic ultrasonic wave generator in an early time and to protect said device by detecting ultrasonic wave transmission current, and judging the presence or absence of any abnormality from the frequency components and magnitude of the waveform of said current. CONSTITUTION:When electric charge is charged from a high voltage DC power source 1 to a capacitor 3 for charging and discharging through a resistor 2 for charging and a switch 4 is closed, the charged electric charge is discharged through a damping resistor 5, a coaxial cable 6 and a signal transmission coil 7. The signal current in this time is detected with a current probe 9, and said current is inputted via a band pass filter 10 and a detector 11 to a comparator 12. In the comparator 12, the current is compared with a threshold value, and when the input signal is larger than the threshold value, a control circuit 13 alarms and displays the generation of abnormality and interrupts the high voltage DC power source 1 in order to protect an electromagnetic ultrasonic wave generator 8.