PROBLEM TO BE SOLVED: To allow accurate detection of surface defects and surface layer defects of a material even when the material such as a steel plate is being transported or moved.SOLUTION: When a thin steel plate is taken as an example of a material, a defect detection system for the steel plate, which is for detecting surface defect and surface layer defect 101 of a thin steel plate 100, comprises a heater 1 for heating the surface of the thin steel plate 100, a infrared thermography camera 2 for obtaining thermal image data of a surface region (inspection area S) during or after heating by the heater 1, and a detector 3 for determining a variation amount of emissivity Δε(x,y) from a surface temperature given by the thermal image data obtained by the infrared thermography camera 2 to detect presence/absence of a defect. In obtaining the thermal image data of the inspection area S by the infrared thermography camera 2, heat energy emitted from the heater 1 is prevented to enter the infrared thermography camera 2 because of its reflection and the like at the steel plate 100.