To provide a decomposition treatment device for waste in which agitation efficiency can be improved and the entire device can be miniaturized.

A flight member 10 is provided with a sliding member 21 connected to an agitating blade 17. Housing guide rails 26, 27 rotate the flight member 10 and displace it to a non-projecting state by making a sliding member 21 slide on an agitator body 5a when the flight member 10 moves from the front side in the agitator advancing direction to the rear side. A sliding guide rail 28 is provided for rotating the flight member 10 to displace it to a projecting state by making the sliding member 21 slide in a lace other than the front side in the agitator advancing direction.