To allow a sealed letter to be sealed with a pressure bonding roller which is smaller than a conventional one in a sealed letter preparation device that seals by folding an enveloped paper with a pressure-sensitive adhesive together with its contents.

The pressure-sensitive adhesive 106 is provided on both edges in the width direction of the envelope paper 100, and a remoistening glue 107 is provided inside both edges. Water is added to the remoistening glue upon forming an envelope 50 by folding the envelope paper wrapping the contents, the entire surface of the envelope is bonded with the remoistening glue by applying a pressure with low-pressure rollers, and the pressure-sensitive adhesive 106 is bonded by applying the pressure to only both edges of the envelope 50 with top-and-bottom-grouped left-and-right pairs of pressure rollers 80 and 80 further narrower in width. Since a high pressure required to bond the pressure-sensitive adhesive is required only for both edges of the envelope, the pressure rollers 80 may be small.