An envelope processing apparatus for enclosing an enclosure into an envelope conveyed to an enclosing position includes a conveyance roller that conveys the envelope to the enclosing position. A first envelope length calculator calculates a first envelope length of the envelope in an envelope conveyance direction in which the envelope is conveyed in a close state in which a flap of the envelope closes. A flap opener opens the flap while the conveyance roller conveys the envelope. A second envelope length calculator calculates a second envelope length of the envelope in the envelope conveyance direction in an open state in which the flap opens. A controller calculates a flap length of the flap in the envelope conveyance direction based on the first envelope length and the second envelope length. The controller controls enclosing of the enclosure into the envelope based on the flap length.