Abstract: A device for opening an envelope is disclosed. In the envelope opening device of the present invention, when a user merely inserts an edge part of an envelope into a guide slot of a casing and pushes or pulls the envelope, the edge part of the envelope is precisely cut by a cutting means to a predetermined length while the edge part is supported and guided by a guide wall, which is provided inside the guide slot, and while the envelope is held and supported by a holding means such that it is prevented from undesirably moving. Therefore, the present invention makes it possible to precisely cut the edge part of the envelope to a minimum length. Furthermore, because the present invention does not require the user to pay much attention to the length of the edge part to be cut, there is an advantage in that the opening process is simplified.