To provide a water tight device for a gate capable of sufficiently applying the pressing force of a seal member for preventing the jet flow against the gate.

This water tight device 21 is constituted to seal the space between the opening 4 at the discharging water passage 3 formed in a bank body and a gate body 12 freely oscillatable inside the vertical surface for opening for closing the opening part 4. The device is made up of a first seal member 23 provided to the circumference of the opening 4 and made to be expandable by the water supplied inside a space 22, and a second seal member 24 provided to the upper periphery of the opening 4 and arranged to the outside of the first rubber-made seal member 23. Further, a cylindrical rubber is used as the second seal member 24, and this second seal member 24 is constituted so as to be rotatable around the axis perpendicular to the oscillating direction of the gate body 12.