To automate sealing operation with a simple structure of a small size which is free from noise and is manufactured at a low cost.

This envelope sealing device comprises an air blowing nozzle 7 which blows air to a flap part 4 of an envelope 2 on a base 1 to open its sealing part of the envelope 2, a slider 9 which shuttles back and forth on the base 1 to make its way into and out of the envelope 2 and introduces a disposed enclosure 18 into the envelope 2 and a claw element 26 which is provided, in a slightly freely introducible manner, in the envelope 2 and comes into contact with the rear end of the enclosure 18 introduced into the envelope 2 and further, prevents the enclosure 18 from slipping out together with the receding of the slider 9.