PROBLEM TO BE SOLVED: To prevent water immersion into a seal groove in a coating device having a seal structure formed by the seal groove and an O-ring on an upper end face of an underplate.SOLUTION: A coating device A comprises: an underplate 26 which is a dish-shaped whose outer peripheral edge part stands up； a spacer 34 (cylindrical member) which is formed into an approximate cylindrical shape whose vertical faces are opened, and is assembled in a state that it is mounted on an upper end face of the outer peripheral edge part of the underplate 26； a seal groove 51 which is formed into a continuous recess shape over the whole periphery of the upper end face of the outer peripheral edge part of the underplate 26 and which has a trapezoid cross section in which a width dimension in a radial direction becomes the smallest at an opened edge 52； and an O-ring 55 stored in the seal groove 51, and sticking to the opened edge 52 of the seal groove 51 in a liquid-tight state in a state that the spacer 34 is removed from the underplate 26.
