A mobile crushing machine, equipped with an impact crusher having a separable casing 31 comprising a stationary casing 70 and a movable casing 30, is adopted for casing 31 with the upper end 724 of the stationary casing 70 positioned below the upper end 820 of the movable casing 80 and with the movable casing 80 arranged to permit an operator to collapse the feeding port 31A side of movable casing 80 into the rotation mechanism 39 for transporting the mobile crushing machine The impact crusher is equipped with a gap adjustment device (60).