To provide a seal plate which can follow the distance change between the upper and lower roll axis centers simply and surely for a long period of time without excessively increasing equipment cost and is installed in a side plate opening part in a horizontal treatment tank, which is provided with the upper and the lower rolls and applies a dipping treatment to a strip.

The seal plate for the horizontal treatment tank is composed of: an upper seal plate member 12 having a roll shaft hole 17 to which a shaft of the upper roll 7 is inserted; and a lower seal plate member 13 having a roll shaft hole 18 to which a shaft of the lower roll 8 is inserted. One of the lower end part 19 of the upper seal member 12 and the upper end part 20 of the lower seal plate member 13 has a cross-sectional recessed part 21 in the plate thickness direction and another one has a cross-sectional projecting part 22 in the plate thickness direction to be engaged with the recessed part 21 and the recessed part 21 and the projecting part 22 are formed to be slid mutually in a vertical direction.