PURPOSE: To enhance inspecting accuracy and to decrease the percent defective of products by passing a steel strip through a pair of upper and lower smoothing rollers for lightly pushing down, and thereafter optically inspecting the steel strip on the surface of the smoothing rollers that was passed and lightly pressed as a test specimen.

CONSTITUTION: Smoothing rollers 4a and 4b are provided at the front stage of an optical inspecting apparatus. The roughness of the irregular surface of a steel strip to be processed is made small by lightly pushing down the strip. A noise level is suppressed to the lower value. The response is made sharp. As the smoothing rollers 4a and 4b, winding rollers which suppress the fluctuation of a bus line are utilized. The rollers comprise a pair of upper and lower steel rollers. The steel strip to be passed and processed is lightly pushed down. Thus, the bending of the steel strip 1 and the surface irregularities are smoothed. In a light-scanning and light-receiving surface inspection device 5 for detecting the flaws of the surface strip, a scanning means 2 and a light receiving means 3 are arranged so as to face the rollers 4a and 4b. In this way, the inspection can be performed under the state wherein the steel strip 1 is wound. The steel strip having the high smoothness can be made to be the test specimen. The highly accurate inspection for the surface flaws can be performed.