To provide a cutter for flat paper sheets and folded/piled paper sheets such as sealed envelopes.

The cutter has a body portion 2 having a lower part provided with a passage groove for paper sheets, a handle portion having a hollow portion for fitting a fitting projection 3 of the body portion 2 in a manner that the body portion 2 is fixedly and removably integrated, having a linear lower side extending from the front end side to substantially the center part, having a rear end side provided with a grip portion, and having a passage area for paper sheets formed between stepped parts and the lower side of the body portion 2, a blade body 21 held by the body portion 2 so that a cutting edge 21a is directed toward the front end side, wherein one part of the cutting edge 21a faces the passage area 8, and the other part of the cutting edge 21a faces the passage groove, and a guide body inserted into the passage groove of the body 2, guiding the paper sheets to the other part of the cutting edge by a leaf spring piece 44 and cutting the paper sheets, and cutting the paper sheets pressed toward the lower side by a guide piece 46 facing the passage area by means of one part of the cutting edge 21a.