To provide a bonder cap capable of improving vehicle mountability of a wire harness by suppressing increase in external dimensions of a wire harness when assembled to the wire harness.

The bonder cap includes: a first slit 11 which is formed by cutting off a peripheral wall of a cap body 10 of the bonder cap 1 formed in a bottomed cylindrical shape and cover-mounted on the bonder of a wire bundle from an open end edge 10a over a predetermined length along the central axis of the cap body 10; and a second slit 12 which is formed by cutting off the peripheral wall from this first slit 11 over the suitable length along the peripheral direction of the cap body 10 and enabling peripheral walls 10b, 10c on both sides of the first slit 11 to be bent and deformed in diameter expansion and contraction directions.