A restraining means includes a shaft member projecting laterally from a frame member, and a plate which is fixed to a base frame and in which an insertion hole capable of inserting the shaft member therethrough is formed. The restraining means is configured so that an inner surface of the insertion hole and the shaft member are not in contact with each other within a moving range of a drivers cab allowable by an installation mount, and so that the inner surface of the insertion hole is in contact with an upper surface of the shaft member when the drivers cab is lifted from the base frame beyond the moving range.