which is integrally formed on a lower end of a tubular second shaft 35 by means of bolts 39 the first shaft 29 is coaxially and rotatably inserted into the tubular second shaft 35 the first shaft 29 is also slidable in the axial direction in the second shaft 29 the second shaft 35 is rotatably provided in the housing 5 through a bearing 41 a cover 43 is mounted on the bottom surface of the flange 31 of the first shaft 29 by a screw 45 to cover the bolts 33 the mowing head 3 includes means for changing the mounting position of the first and second cutting disks 7u 7l in the circumferential directions of the cutting disks 7u and 7l the mounting position changing means comprises a positioning member 47 in which means for reciprocatively oscillating the cutting disks is installed further the mounting position changing means includes a drive gear 101 and a driven gear 103 a detailed structures thereof are described later referring to the detailed structure of the positioning member 47 the upper end of the first shaft 29 is rotatably supported to the positioning member 47 through a bearing 49 and the middle portion of the second shaft 35 is also rotatably supported to the positioning member 47 through a bearing 51 the positioning member 47 has a disk shaped plate portion 53 a peripheral portion 55 of the disk shaped plate portion 53 has a circular outer periphery 57 which slidably contacts on an inner surface of the housing 5 therefore the positioning member 47 is provided rotatably in the housing 5 on.