PURPOSE: To obtain an induction heating flaw detector which can detect a flaw with high reliability regardless of the shape.

CONSTITUTION: When a surface temperature variation calculating means 81 calculates a signal SΔT representative of the local temperature variation on the surface of a conductive material 10 on the basis of temperature detected signal ST outputted from a radiated temperature measuring instruments 78a-78d, a flaw decision means 83 whether or not detects a flaw depending upon the absolute value SΔT exceeds a preset decision criterion Th. Consequently, even such a effect on the local surface of the conductive material 10 as can not be detected based on the negative variation of surface temperature can be detected based on the positive variation of surface temperature. This detector can detect a flaw with high reliability regardless of the shape.