A pit gate 15 that is accommodated in a slot 14 and that seals service water retained in a first pit and a second pit in a watertight manner includes a gate main body 21; a sealing member 22 that is provided on a surface opposite to the slot 14 in a gate main body 21 and that seals between the gate main body 21 and the slot 14 in a watertight manner; and an upper pressing clamp 23, an intermediate pressing clamp 24, and a lower pressing clamp 25 that are provided in the gate main body 21 and that moves gate main body 21 toward the sealing member 22 side due to the weight of the gate main body 21 itself.