PURPOSE: To provide the seal structure of the motor-driven toothbrush, in which a projection amount of an annular projection is not enlarged, and the annular projection can be prevented enough from falling out from an annular groove at the time of using the motor-driven toothbrush.

CONSTITUTION: The seal structure is constituted so that an annular projection 20b, and an inner annular groove 20c being adjacent to this annular projection 20b and extending in the peripheral direction are formed on the inside peripheral surface of an end part of a cylindrical case 1 side of a cylindrical seal member 18, the annular projection 20b is fitted and attached to an outer annular groove 9 of the cylindrical case 1, and also, an outer flange for forming the outer annular groove 9 is fitted and attached to the inner annular groove 20c.