Abstract: An oral hygiene instrument according to this invention by means of vibration generating means vibrates by way of a holder member an oral hygiene tool removably attached to the holder member and comprises a cap member having a cap side thread portion which meshes with a holder side thread portion formed at a first end of the holder member which cap member is removably attached to the first end of the holder member and closes an opening in the first end of the holder member, a seal ring fitted on a portion of the holder member or the cap member at the external end of the part where the two members mesh which seal ring is pressed upon by either the cap member or the holder member and seals the gap between the holder member and the cap member within a range of a predetermined angle of screwing of the cap member with respect to the holder member from a late stage to completion of said screwing, and a holder side contact piece and a cap side contact piece mounted in the holder member and the cap member respectively