PURPOSE: To surely and safely open an envelope by housing a cutter blade holding part equipped with a cutter blade in a slide groove so as to be freely slid at an opening part equipped with an almost forked guide formed at one end of the main body of the cutter concerning the cutter for opening envelope to open the sealing of the envelope.

CONSTITUTION: When opening the sealed part of the envelope, first of all, a cutter blade 3 is protruded out of the tip of a main body 2 of the cutter by operating a knob 6 of a slide blade pair 5 with the tip of a finger. In such a state, a tip 16 of the cutter blade 3 is inserted to one corner of the sealed part of the envelope, the finger is detached from the knob 6 after the confirmation of a cut end, and the cutter 1 is moved forward in the direction of the other end while sandwiching the part to be opened with a guide part 9 as it is. At such a time, the protruding cutter blade 3 is pushed back by the resistance of the paper sheet of the envelope with the start of moving of the cutter 1 and temporarily moved back until a holding part 4 of the cutter blade touches a stopper 16 at the other end of a slide groove 8 and in this state, a piece of paper is cut.