An expansion instrument includes an elongated vaginal-insertion member insertable into the vaginal lumen which is adjacent the urethral lumen with biological tissue interposed between the two, an elongated urethral-insertion member insertable into the urethral lumen, an interlock device interlocking the vaginal-insertion member and the urethral-insertion member to allow the insertion members to be brought closer together and brought farther apart, and a vaginal-side restriction that restricts the positional relationship, with respect to the biological tissue, of the vaginal-insertion member after being inserted into the vaginal lumen. The vaginal-side restriction is on the vaginal-insertion member and has a configuration such that in a state where the vaginal-insertion member is in the vaginal lumen, the urethral-insertion member is in the urethral lumen, and the positional relationship has been restricted by the vaginal-side restriction, moving the vaginal-insertion member farther away from the urethral-insertion member widens the biological tissue in the moving away direction.
