To provide a projection cutter which can neatly cut a projection such as a rail provided on an aluminum sash, in a manner of leaving little distortion at a cut portion while saving labor, and to provide a projection cutting method.

The projection cutter for cutting and removing the long projection 2a projecting from a projection cutting object 2, is formed of a rotary cutting blade 3, a cutting reaction force bearing member 4, a first cutting mechanism 5, and a moving mechanism 6. The rotary cutting blade 3 can cut the projection 2a by engaging in one surface of the projection 2a. The cutting reaction force bearing member 4 abuts on the other surface of the projection 2a at the time of cutting the projection 2a by the rotary cutting blade 3 and positively bears reaction force. The first cutting mechanism 5 moves the rotary cutting edge 3 and the cutting reaction force bearing portion 4 located on both the front and rear surfaces of the projection 2a independently from each other, so as to relatively approach to each other, and engages the rotary cutting blade 3 in the projection 2a, to thereby cut the projection 2a. The moving mechanism 6 arbitrarily moves the cutting blade 3 and the cutting reaction force bearing member 4 in a longitudinal direction of the projection 2a.