PURPOSE: To keep the border-detection accuracy and prevent the malfunction and damage of a moving operation means by preventing the error in the detection of the presence of grass caused by cut grass, etc., attached to the mower.

CONSTITUTION: This apparatus is provided with a grass detection means 22 to detect the presence of grass at prescribed intervals along the lateral direction of the machine body by grass-detection sensors S1, S2, S3, S4 to detect the presence of grass by projecting a grass-detection signal on the grass in front of the machine body and a border-judging means 100 to determine the border between the untreated ground A and the treated ground B based on the information transmitted from the grass detection means 22. The grass-detection means 22 is positioned above a grass-height restriction means 11 placed at the front part of the machine body to restrict the upward protrusion of grass by guiding the grass downward and is placed behind the front end of the height-restriction means. The grass detection means 22 is provided with a moving operation means 23 to move the grass-detection sensors S1, S2, S3, S4 along the lateral direction of the machine body.