PURPOSE: To make it possible to perform accurate horizontal control with im proving reading accuracy of an angle sensor by amplifying relative rotation angle between left and right lower links and measuring.

CONSTITUTION: A first detecting arm 17 is rotated around a horizontal arm shaft 11 in same angle as rotation of a left lower link 2 following to the rota tion of the left lower link 2 and a second detecting arm 18 is rotated around the horizontal arm shaft 11 in same angle as rotation of a right lower link 3 following to the rotation of the right lower link 3. Therefore, a relative rota tional angle between the both detecting arms 17 and 18 is always coincide with a relative rotational angle between the left and right lower links 2 and 3. In a case of generating a slight difference between rotations of the left and right lower links 2 and 3, an amplified rotational angle is able to be measured by an angle sensor 16, whereas a relative rotational angle between the detecting arms 17 and 18 is vary small. By said method, reading accuracy of the angle sensor 16 is improved and a slight inclination of left and right is able to be detected.