To provide a waterway installation type dreg crusher reduced in manufacturing cost.

The dreg crusher 1 comprises an inner rotation body 3 having a spirally wound screw blade 2 detachably mounted with a crushing blade 21, and an outer body 4 presenting semi-annular shape from the plane cross section view toward the lower reaches and provided with ring bars 17 at vertical spaces to form slit openings 18, wherein the crushing blade 21 are rotated to pass through the slit openings 18 to crush dregs. The crushing blade 21 and the slit openings 18 are formed only on the lower side of the inner rotation body 3 and outer body 4 respectively, and a dreg catching screen 19 is provided on the upper side and downstream side of the outer body 4.