PURPOSE: To set a seat low by projectingly installing a cylinder upwardly at an angle of about 20° for an installation standard surface in the nearly horizontal direction and forming a swing shaft hole of a lift arm above the lager diameter position than the bore diameter extension part of the cylinder at the rear part of a hydraulic cylinder case.

CONSTITUTION: In the upper part of a transmission case 1, a hydraulic cylinder case 2 is installed, and a seat 3 is arranged above the case 2. In the transmission case 1, the case 2 is installed on an installation standard surface A-A which is worked nearly parallel. In the front part of the case 2, a cylinder 4 part is projectingly installed forwardly and upwardly at an angle of about 20° for the installation standard surface A-A. Both the side edges of a swing shaft 25 which is supported on two points in a swing shaft hole 6 opened on the rear part side wall of the case 2 project outside the rear part side wall of the hydraulic cylinder case 2, and the left and right lift arms 5 and 5 are installed at the projecting edge part. Since the cylinder 4 and the swing shaft hole 6 are projected in assortment at the front and rear parts of the case 2, the space for installing the seat 3 can be made flat and low as a whole.