A bulldozer is provided with a boat shaped main frame and a substantially U-shaped gate frame as a frame structure that mounts a cap having a rollover protection structure (ROPS) and forms the skeleton of the bulldozer. The gate frame is installed on the upper portion of the main frame, has a pair of left and right columns that are disposed along a substantially vertical direction, and has a beam that connects the upper ends thereof. Mount units for mounting the cab and stopper units for firmly grasping the cab are arranged on the upper surface of the beam.