Abstract: An automatic envelope sealing device wherein envelopes of different sizes can be sealed without adjustment any component of the device. A size of an enveloped supplied into the device is detected by a sensor, and in response to a signal from the sensor, a movable stopper is alternatively extended into a plane in which the envelope is to be fed or retracted in order to stop the large size envelope thereat or retracted therefrom to allow the small size envelope to be fed until it is stopped by a fixed stopper. A flap of the envelope is then bent positively and paste in a controlled amount is applied to the flap whereafter the envelope is fed reversely until it is discharged outside the device. During the reverse feeding, the flap is pressed to be adhered to the envelope by cooperating rollers which act as feeding means.