An envelope processing apparatus conveys an envelope to an enclosing position and encloses an enclosure on which specific information is formed at a formation position. The enclosing device includes: an envelope conveyor to convey the enclosure to the enclosing position to insert the enclosure into the envelope; a sensor to acquire, as an image, a structure of the envelope during conveyance, and acquire transparent-portion information indicating a position of a transparent portion in the envelope; an envelope holder to hold the envelope in a state in which the enclosure is insertable; and control circuitry to output position adjustment information for adjusting a position of the envelope in a width direction of the envelope at the enclosing position or a position of the specific information in a conveyance direction of the envelope, based on relative positions between the formation position of the specific information and the position of the transparent portion.