PURPOSE: To improve the safety and workability in a work by surely protecting a cabin from a dropped-down article during the work and securing the view field from the cabin.

CONSTITUTION: A head guard body 34 covering the upper side of a cabin 4 is constituted of a fixed guard part 35 having an accommodation space 35A formed inside, and a movable guard part 36 which is accommodated inside the accommodation space 35A of the fixed guard part 35 and permits the horizontal shift in the longitudinal direction and the movable guard part 36 is positioned between the retreat position for the accommodation into the accommodation space 35A of the fixed guard part 35 and the bulging-out position for the projection to the front side of the cabin 4. Accordingly, when the movable guard part is positioned at the bulging-out position, the cabin can be surely protected from a dropping-down article, and the safety in the rock crashing work, etc., can be improved, while in the case where the movable guard part is positioned at the retreat position, the view field on the aslantly front upper side of the cabin 4 can be secured, and the workability in the work having the low dangerousness for the dropped-down article can be improved drastically.