The scanning method with GM survey meter is useful for detection of surface radiological density. The detection ability is varied with several parameters; those are the distance between the sample and the window of GM tube, the scanning speed and the background dose rates. We made an experimental system. In that system, a radiation source can be moved at any constant velocity with a linear scanning device. The obtained data shows that the change of count rate has some patterns due to each scanning velocity. The count rate increased with the source coming closer, and decreased with the source moving away. The maximum count rate under the scanning condition, those are 5 mm distance and 50 mm/s scanning velocity, point source on a stainless steel plate was 23% of the count rate on the fixed source. The detection ability in various scanning conditions was evaluated from the measured data. As a result, it was confirmed to be able surely to detect the lower level of the surface radiological density limit by means of the scanning condition applied to the routine work, 5 mm distance and 50 mm/s scanning velocity.