To firmly support a glass door so as to improve durability by forming a cabin door by transparent glass, and fitting a reinforcement frame including a longitudinal frame for connecting the rear parts of an upper frame and a lower frame to each other to the inside of the transparent glass.

Front supports are erected on both right and left sides of the front of a floor 11, and rear supports are erected on both right and left sides of the rear. A roof 13 is placed on a connecting frame 12 provided between the upper end parts of both supports to construct a cabin. A reinforcement frame 4 comprising an upper frame 15 laid along the longitudinal direction, a lower frame 16 laid along the longitudinal direction in the middle of a door 3, and a longitudinal frame laid in the vertical direction for connecting the rear parts thereof to each other is fitted to the inside of the door 3 formed by a transparent glass. The reinforcement frame 4 is fitted to a region extended from the end edge of the door 3 inward, and the door 3 is pivoted on the rear supports in such a manner as to be freely opened and closed. Thus, strong impact load is not concentrated to prevent breakage of glass.