To improve maintenance property by modifying an expansion mechanism of a crawler and reduce cost by decreasing weight, in a construction machine provided with a crawler expansion device.

A lower frame 1 comprises a cylindrical body 2 used for holding an upper structure turnable, axle beams 3f, 3r that are fixed back and forth of the cylindrical body 2 and extended right and left, and crawler frames 4L, 4R mounted to respective both ends of the axle beams 3f, 3r slidably right and left. A retract cylinder 25 is mounted to the cylindrical body 2, and is a double-acting cylinder in which pistons and piston rods are provided right and left of a cylinder tube. Tips of right and left piston rods 26L, 26R are passed through the substantially central parts of the right and left crawler frames 4L, 4R, and are connected with brackets 19L, 19R fixed to the outside surfaces of the crawler frames 4L, 4R.