This paper describes the influence of physical properties of workpiece material on the difficulty in laser piercing through measurement of the absorption factor change with extension of the depth of hole pierced by a YAG laser. To measure the absorption factor, which is defined as the ratio of the power absorbed by the workpiece to the incident laser power, the inverse problem solution was utilized. Regardless of material properties, the absorption factor increases with the increase of the depth of the keyhole, and after reaching about 100%, the increase is stagnated and the absorption factor is saturated at 100% followed by a rapid decrease after the penetration. Thus it was concluded that the absorption factor in laser piercing is determined not by the intrinsic reflectivity of workpiece material but by the depth of the hole being processed. Consequently, difficulty in cutting copper workpiece by laser beam results from the high thermal diffusivity of copper rather than its reflectivity.