PURPOSE: To reduce extremely generation of a noise based on a mechanical driving and switching of a switch, and to execute an exact flaw detection by applying an exciting signal by which the sum of its terminal voltage becomes constant hourly, to a detecting coil arrayed by a specified relation, and discriminating a flaw basing on a detecting signal which is the sum of the terminal voltage.

CONSTITUTION: A modulating signal generating circuit 7 gives successively a pulse whose phase is shifted, to each modulation circuit 4. A time shift of each pulse is equal to the time obtained by dividing the width T0 of this pulse by an overlapping number of a detecting coil 2. Each modulation circuit outputs an exciting signal successively at an interval equal to a time difference. The exciting signal is applied to each detecting coil 2 through a variable impedance element 3. Terminal voltage of the detecting coil 2 is synthesized by an adding circuit 8, and as a result, in case a flaw, etc. do not exist on a steel plate 1, when the detecting coil 2 is scanned once, the adding circuit 8 outputs a detecting signal. This detecting signal is applied as one input of a differential amplifying circuit 9. The differential amplifying circuit 9 outputs a differential signal which subtracts a correcting signal being the other input, from the detecting signal.