To solve a problem wherein an envelope opener cannot cut a large envelope, etc. because of its structure, the strength of a cutting blade is controlled by a cam and hence it has to be adjusted at every time, which is troublesome, and the opener cannot cut a thick stuff, and it is difficult to know the cutting position of an envelope, etc.

In front of a body formed with a guide to insert the envelope in a horizontal direction, a spring that moves up and down a cutting blade holder whereon the cutting blade is removably mounted is provided. A pressing tool for pressing the cutting blade holder downward is also provided.