PURPOSE: To clearly observe the surface state of a hearth roll in a high- temperature furnace by photographing a picture with only an external strobe light source which radiates on object with light having higher illuminance than in-furnace background light.

CONSTITUTION: The hearth roll 1 as the object is heated to high temperature in a continuous annealing furnace wall 2 and the surface of the roll 1 is detected by being detected by a camera 6. A strobe 5 is capable of radiating the surface of the roll 1 to larger than the illuminance of the background and also positioned so that when the surface of the roll 1 is radiated with light having higher illuminance than the background light, reflected light from the surface of the roll 1 never enters the visual field of the camera 6 directly. Further, the lens 7 of the camera 6 is set to 280mm focal length, a 157H×105Vmm determined field angle, and F8 diaphragm, the light emission time of the strobe 5 is set to ≤1/(33600)sec, and an ASA1600 film is used. Thus, the surface condition of the hearth roll can be observed in the high temperature furnace clearly.