An envelope processing apparatus that inserts an enclosure into an envelope includes an envelope conveyor, a flap opener, an enclosure pusher, and circuitry. The envelope conveyor extends in a vertical direction and conveys the envelope to an enclosing position via an envelope conveyance passage as a passage in which the envelope is conveyed. The flap opener opens a flap of the envelope. The enclosure pusher pushes the enclosure toward an opening of a body of the envelope conveyed to the enclosing position with the flap open. The circuitry changes a pushing amount of the enclosure into the envelope and adjusts the pushing amount based on the opening of the body of the envelope within a specified range from an inside to an outside of the body of the envelope.