It is said that the road surface has significant effects on road noise. For a proper evaluation of the effect of the road surface, it is important to develop a measurement method of the absorption coefficient for oblique as well as vertical incidence. A measurement method was developed that can be used in a hemianechoic room. A long strip of test material approximately 2 m in length and 0.5 m in width is placed on the floor of the room. A test noise is generated by a line array of loudspeakers which is placed above the floor and parallel to the test material. The distance from the material to the loudspeaker array is approximately 2 m. The position of the loudspeaker array is moved on the circumference so that the incidence angle may be changed while keeping the radius constant. The incident power is measured beforehand in an anechoic room which is proportional to the sound pressure squared. The absorbed power is measured by use of the sound intensity method. The measurement points of the intensity are on a smaller semicircle (approximately 0.5 m) with the same center as the test material. The absorption coefficient is given by the ratio of the absorbed power to the incident power.