Active infrared thermography is a method to detect concrete spalling by heating the surface of the concrete using irradiation devices, such as halogen lamps, xenon arc lamps and far-infrared irradiation devices. We carried out irradiation tests of plain plate specimens using all of these devices and compared the results with those obtained by transient heat-conduction analysis. Specimens with artificial voids were then prepared for active infrared thermography using a xenon arc lamp and subjected to a laboratory test. A field test carried out on an actual viaduct to prove the effectiveness confirmed that active infrared thermography could detect concrete spalling accurately.